Thomas and Atlanna getting together is an alternate tale of the romance between an otherworldly female and a human male, last seen in Wonder Woman (2017). Atlanna and Thomas are what Diana and Steve could have had (in fact both couples share a romantic moment in snowfall).
Their child, a half-breed superhero, is an alternate tale of the issues a child of two worlds faces in Man of Steel. While in Man of Steel Clark ultimately chooses humanity over Kryptonians, Aquaman has Arthur learning to ultimately accept both sides of his heritage as a person of the land and the sea (it helps that his people from the sea have not gone extinct and don't all seek to destroy humanity).
When King Orm is suddenly thrown from water to air he is shown coughing out water as he tries to breathe. It may seem odd that he briefly seems to be drowning, but it's actually a rarely-addressed moment of Reality Ensues for a being that can breathe both water and air: they'd have to empty their lungs of water before they can start breathing air, and vice versa when they return beneath the ocean.
Neither Arthur nor Mera are ever shown coughing out water from their lungs whenever they go back into the air, but this is more fridge brilliance. Arthur was practiced at moving between land and sea, so perhaps he developed a method to quickly expel both air and water from his lungs when moving between. Mera, while seemingly not fond of the surface, has been shown to move between both sea and surface on at least a few occasions, such as when she summons Arthur. Orm, however, has been in the sea his entire life.
Orm spent his life training, so part of that training should involve surface breathing out of water. But he was taken off guard the first time, the second time he adjusts easily.
Atlanna also coughs up water after Thomas rescues her and takes her into his home. Injured and unconscious (and maybe having enough oxygen suspended in the water in her lungs she didn't need a new "breath" immediately) there was a delayed reaction, but she did expel water before breathing air (Thomas states "You weren't breathing!")
On a sillier note, this also explains why Thomas can still drink Arthur under the table in a chugging contest despite being a normal guy compared to his son the super-being. Being able to switch back and forth between breathing water and air probably means that Arthur never had to learn how to hold his breath, seeing as he never had a reason to. As such Arthur probably can't really hold his breath for more than a minute(if that) and as such can't hold out in the middle of drinking a beverage he can't breath, while Thomas, the normal human who had to learn to hold his breath to swim, can probably go awhile at it and doesn't have to stop to breath in the middle of his drink.
Why did Orm fight Arthur outside of water where Arthur has the advantage? Because now that Arthur has the trident and can command all of the sea creatures along with the Karathen, there's no way he could win underwater. The fact that Orm tried to instigate a one-on-one fight shows that he acknowledges that he can't beat Arthur even with his armies underwater so he had to risk fighting Arthur on the surface in spite of the disadvantage.
Or it could have been dressings on his face after he took the concrete bolas to the face.
Black Manta is given a large plasma rifle that he (dedicatedly) converted into a diving helmet with optic lasers. Kane shows early on that he prefers blades, so this extensive modification makes sense for him; it supplements his preference for CQC by allowing him to retain an effective long-range weapon while still leaving his hands free to wield his knives and grapples.
And since firearms indirectly caused his father's death, he may have developed an aversion to using them.
Besides, knives are a tradition of the Kane family, as seen from the blade his father gave to him.
The revelation that Atlanteans don't need a water bubble to talk to one another paints Aquaman's interaction with Mera in Justice League (2017) in an entirely different light. Mera didn't create the bubble so she could talk to Arthur, she did it so that they would not be overheard by anyone walking in.
Wetsuits are the standard Atlantean wardrobe, perfect for underwater living.
Dr Shin getting publicly humiliated for claiming that Atlantis is real may seem a bit out of place considering by now the world is aware of the existence of aliens and meta-humans, but there's a Genius Bonus behind that. Atlantis was a fictional nation derived by Plato, similar to how H.P. Lovecraft came up with R'Lyeh. Every other paranormal entity that's appeared in the DC Universe stems from folklore.
Mera doesn't wear her crown throughout the film, mirroring her conflict with royalty about its decision and drive to wage war on the surface. It's not until Arthur proves himself worthy of the trident that she wears her crown with pride, siding with a member of Atlantean royalty who actually means well for all.
Her confrontation with Orm and Nereus mirrors her evolving attitude. She takes a confrontational approach, snapping at Orm in a child-like manner. Later on when she meets her father after seeing the Earth people and after Arthur proves his worth, she is calmer and more reasonable and points out that the war will just cause loss and destruction to all, including Atlantis itself, acting like a proper queen.
The Trench are silent and deadly. Stealth matters to fish-like predators with light-sensitive vision who rely on sound/sonar to navigate.
Arthur feeling more sympathy for Black Manta over his father's death and the anger he feels towards Arthur for not saving him is understandable since he very nearly lost his own father Thomas (and would have if not for Mera).
Analyzing King Nereus after the film's done actually paints a compelling picture of him as a surprisingly cunning and savvy politician, far more so than Orm. While Orm is using all of his skills at political theater and manipulation to try and forge his alliance, none of it ever tricks Nereus, even the False Flag Operation, as pointed out by Mera. Nereus shares Orm's vision of turning the might of Atlantis upon the surface world, but is content to allow Orm to be the public face and main pressure behind the uniting of the kingdoms and to officially fall for his charade of a surface world strike, putting all the risk and blame on Orm. He's sliding into the Kingmaker Scenario casually and at Orm's invitation, with his daughter's engagement to Orm securing his family's ascension to the throne of a united Atlantis through her... until she runs away to help Arthur.
The one manipulation of Orm's that he does seem to fall for is Orm's promise to spare Mera, when he fully intends to kill her... unless he knows that a possibility, and is merely ensuring that Orm's word is on the record in case his Action Girl of a daughter (who he, at minimum, knows was capable of tossing Orm around with her powers) manages to return to him; with Orm's official will being to spare her, all he has to do is publicly assert he's satisfied with her return, and he's got a decent chance of preventing her murder.
And notice, when Arthur arrives at the final battle and turns the tide with the trident of Atlan, Nereus is able to disentangle himself from his alliance with Orm toot-sweet, leaving Orm on his own to suffer the consequences of his failure. And while Nereus clearly didn't plan for his daughter to fall for Orm's rival for the throne... that still happened, and he's still in the same political position he wanted in the first place, though his goal of attacking the surface world has been sacrificed in exchange.
How did Atlanna survive the Trench? By getting to the shores/jungles of the Hidden Kingdom. Atlanteans can breathe underwater, but only their royal members can breathe on the surface. Any normal Atlantean would be stuck in the Trench, while royal members like Atlanna, Arthur and Mera made it to the Kingdom's shores, though they'd be trapped with no way out.
Orm gives Mera a bracelet, saying it was his mother's. As it was a tracking beacon, he may have been telling the truth, because how did the Atlantean guards track down Atlanna at the lighthouse? And while she had switched to wearing human clothes, the bracelet was stored with her Atlantean things somewhere.
The film is mainly concerned with the dynasty of the Atlantean monarchy, but Black Manta is from a dynasty of pirates, starting with his grandfather, who was a frogman in World War II nicknamed "Manta" for his stealth and turned pirate after he was mistreated in the U.S. Black Manta's destiny to be a great supervillain borne of this dynasty is right there in his name: he's David Kane, son of Jesse Kane. In the Bible, David, son of Jesse, became a great king of Israel.
The full wardrobe of King Atlan is split between Orm and Arthur: the former wears the crown in his meeting with Arthur, while Arthur himself gets the full gear from Atlan himself at the Hidden Kingdom.
Orm being a better fighter in the sea and Arthur a better fighter on the surface is actually incorporated into the choreography of their fights with each other. Arthur really, *really* wants to use leverage and his superior strength to throw Orm around, which Orm can easily evade in water, where his greater finesse in swimming allows him to outmaneuver his brother. When fighting underwater, it's noticeable that Arthur early on tries to grab Orm, only for Orm to easily dodge and begin dominating the fight. When fighting above water, Orm is clearly driven back at Arthur's leisure.
Orm is blond and clean-shaven, which is Aquaman's classic look in the comics, while Orm in the comics is dark-haired. The contrast is deliberate, it gives Orm the classic Atlantean ruler look but deconstructs it by having him be the villain.
Mera eating a rose makes sense when you consider that for people who live under the sea, the diet is bound to be raw meat or plants, since you can't cook underwater.
This was seen earlier with Atlanna: she ate raw fish out of the bowl, and when Thomas brings her a warm drink she has to follow his motions and blow on her own drink to cool it down enough.
All members of Atlantean royalty have tridents: Atlan, Atlanna, Orm, Nereus, the Fisherman King and the Brine King.
The Fisherman King's trident is encrusted with jewels, but it's far too pretty to be used in a fight.
The Brine King's trident is a bare trio of metal spikes, but it is a good weapon.
The tridents used by the normal humans combine functionality and design. Especially Atlan's trident, which is simple but made of gold, and Nereus's trident, which has a secret blaster weapon.
Although the Trench are evidently too degenerate to wield weapons, their hands have three talons, maintaining the triple-pointed-weapon pattern in any of them that might descend from their ancestors' king.
Arthur being the first Atlantean in millennia to be able to talk to the ocean creatures may seem strange because he's only half-Atlantean but it actually makes sense. The Atlanteans changed drastically over time into 7 different forms. It's likely that even the human looking Atlanteans were different from the Atlanteans of King Atlan's time and remember that it was during his time that Atlantis sank beneath the oceans. Since Atlanteans are really just offshoots of humanity or another branch of humanity, by being a mix of land human and ocean human, Arthur was probably the closest the what the Atlanteans were during King Atlan's time.
Aquaman blocking bullets with a detached hatch at the start of the submarine battle may seem strange, since in that same sequence we see him get shot with a pistol and later a grenade launcher and No-Sell it. But note that he frequently tried to avoid getting shot in various ways. As an Atlantean, accustomed to the pressure of the bottom of the ocean, his skin is super-dense and difficult for surface weapons to penetrate... but, unlike Superman, who is truly Nigh-Invulnerable, Arthur is merely extremely resilient. Even if it doesn't do fatal or dangerous damage, being shot likely hurts.
It's probably more pragmatism that he makes himself a bigger target, after all, they are shooting at him in a submarine a stray bullet in the wrong spot would have brought water in, making it more difficult to save the hostages.
Arthur wonders how, despite his powers, his father is able to drink him under the table. One of the side effects of alcohol is dehydration, it dries you out on the inside. It makes perfect sense that a metahuman with water based powers would have a lower tolerance for alcohol than his normal human parent.
A small one, but we find out that Arthur reacted badly to Bruce Wayne saying "I hear you talk to fish" because that's what his childhood bullies used to taunt him with.
During the attack on the Brine Kingdom, we see one of the sea-reptile mounts chomping down on a Brine soldier. A few species of real-world mosasaurs are thought to have been primarily shellfish-eaters, based on the shape of their teeth.
The Trench people attacking surface ships in the open seas with a murderous frenzy, makes one wonder how many humans have fallen prey to them considering how two superstrong Atlanteans were forced to flee for their lives.
What exactly happens to Murk afterwards? Given he can't breathe on the surface he's pretty much stuck in that toilet bowl until someone rescues him. If he's rescued at all.
It's actually worse; that water only has a finite amount of oxygen for him to intake, so unless he figures out how to operate the lever and get fresh water in the bowl, he's gonna suffocate pretty fast either way.
The Karathen being revealed as an intelligent, sapient creature, but unable to communicate with anyone other than Arthur, or a king from centuries ago. Was it a case of And I Must Scream for her? Was she on the verge of madness, as she was heard speaking even before Arthur revealed he could understand her, and she was just babbling to herself?
Orm's "warning" tsunamis sent against the surface world likely caused a huge amount of casualties and ironically would have likely caused the type of environmental disasters (sewage run-off, oil spills, etc.) that he used as justification for his campaign. Also, it is likely that one of the reasons why none of the other superheroes appear in the film is that they probably are too busy dealing with the aftermath of the attacks.
While it was toned down for the movie, one has to wonder what would happen to an 88,000 ton aircraft carrier as it gets tossed around in conditions it was never designed for, let alone the destroyers that were also washed up. How many of the thousands of crew were severely injured or even killed as their ship got tossed around... all without ever knowing what was going on, and being powerless to do anything about it?
It is so much worse than that. That was a US supercarrier. Those are nuclear. They better hope that reactor stays contained or it might render half the city uninhabitable.
And since the Atlanteans cleaned out the whole sea, this might be a good time to be reminded that the US has 12 of them, and numerous more nuclear submarines. Describing this as a a potential ecological catastrophe might be an understatement...
The overlooked casualties of the Final Battle. The Karathen's entrance may have dealt more damage to the Brine's forces than King Orm's alliance and may have triggered a sea-quake that resulted in a tsunami. The numerous sea life being commanded to fight the military would make marine conversationalists weep from their deaths, as many species shown are already endangered or worse.