Despite being invertebrates, animals without skeletons in Real Life (such as octopuses, jellyfish, etc.) are shown to have them in fiction.
Marine animals that are bottom dwellers in real life (such as crabs and starfish) are shown to be able to swim around in the water in fiction.
If there are talking animals, creatures like seals, and sea turtles can still talk underwater despite not being able to breathe it.note Actually Truth in Television for cetaceans like whales and dolphins: while they are unable to breathe underwater just like any other mammal, they have specialized vocal organs that allow them to communicate with sound even while holding their breaths underwater.
Despite the name of the trope, Toothy Bird also applies to any marine creatures that have teeth in cartoons despite the real life counterparts being toothless.
Due to the Pun-Based Creature trope, a lot of sea creatures will be portrayed more towards resembling the thing that they're named after. Examples include:
Tiger sharks are often orange with black stripes and demonstrate feline-like behavior. note Although they aren't orange, younger Tiger Sharks do in fact have stripes but they fade away by adulthood.
Jellyfish actually producing jelly, whether they lactate or have to be killed for the jelly.
Brine shrimp being advertised as some sort of sapient primate-like pets, usually under the name "Sea Monkeys" (though it could use a Sea Aping-style of naming due to the namebeing trademarked).
Writers tend to confuse freshwater and saltwater with each other, because a lot of the times, a marine animal is placed in a freshwater body (such as a shark in a lake). In fact, aquatic animals may not even need water to survive; any form of liquid will do!
Fish
Even though fish (except sharks) can't blink in Real Life, they are shown being able to blink in cartoons anyway.
Most animated fish (not counting the realistic-looking ones) will look absolutely nothing like actual fish species.
Sharks and other fish are often drawn with some of their fins missing. Usually only the pectoral, dorsal, and caudal (tail) fins are shown while the pelvic, adipose, and anal fins are left out.
Piranhas are portrayed as ravenous, threatening fish that are especially dangerous when they form schools, as they devour everything on their way in alarming speeds. However, that's only true in real life if they're sufficiently starved (usually done by manipulation).
Though not a marine fish, but rather a freshwater fish, goldfish are often portrayed as braindead and forgetful due to the urban legend that goldfish have a 3-second memory.
It is commonly stated that sharks need to keep swimming in order to breathe, in a process called Ram Ventilation. While it's true some sharks can't stop swimming, others can still breathe when motionless through Buccal Pumping, by moving the floor of the mouth so the gills can extract oxygen from the water.
Sharks being portrayed as relentlessly attacking and eating humans. In reality, sharks very rarely attack humans since they prefer prey that provide the most nutrients (which humans don't have enough of). Even then, a shark's attack on humans is usually (but not always) due to mistaken identity, like a surfer looking like a seal from below. The reason sharks bite is because that's their only means of figuring out what something is.
The popular portrayal of the blobfish is actually based on the result of it being taken onto land, suffering tissue damage from the pressure. They actually look less ugly in their natural habitat at extreme depths.
Cetaceans
Easily the most egregious example is when writers mistake cetaceans (whales, dolphins, and porpoises) for fish.
Sperm whales are often depicted with a wider head, shovel-like jaws instead of narrow jaws, upper teeth, belly lines similar to those of baleen whales, and the blowhole located on the top of the head instead of the left side of the snout.
Baleen whales are usually drawn with a single blowhole like a toothed whale. In real life, they have two blowholes.
Whales (sperm whales in particular) were most portrayed as Monster Whales: violent, ravenous beasts that devoured everything they could and destroyed whatever they couldn't, complete with tales abounded of unlucky people being Swallowed Whole by whales, to the point that "belly of the whale" became a metaphor to describe a dangerous and difficult to escape situation. This depiction of them is mostly out-of-date, due to science marching on and us learning more about whales.
Orcas incorrectly being called "killer whales", despite the fact that they are dolphins. Orcas are under the suborder Odotoceti, otherwise known as toothed whales, while whales are classified under the suborder Mysticeti, otherwise known as baleen whales. Despite the fact that orcas are the largest toothed whale, they're not quite big enough to be considered a baleen whale, as well as the fact that they have teeth like all other dolphins while baleen whales have filtering structures in their upper jaw that dolphins do not have.
Confusing porpoises with dolphins. Dolphins have conical teeth and curved dorsal fins, whereas porpoises have spade-shaped teeth and triangular dorsal fins. Also, porpoises have shorter mouths than most dolphins.
The Friendly, Playful Dolphin trope depicts dolphins as, well, playful, friendly creatures. However, the dolphin, much like the human, is known to engage in physical violence for sport, as opposed to merely food or self-defense. Gang rape is common among their pods, too. Furthermore, dolphins are inherently wild animals; it is possible to teach them to be friendly and playful, but there have been many instances in which captive dolphins have actually killed or, at the very least, severely injured humans, including their own trainers. Finally, bottlenose dolphins can weigh more than 1400 pounds (635 kg), and hold their breath far longer than humans can, so even when dolphins are being friendly, they can be extremely dangerous. However, modern media have caught onto this and there have been more portrayals of dolphins as vicious, aggressive creatures.
Any large cetacean being accompanied with the "song" of humpback whales unless if it actually is a humpback. Especially jarring when orcas are shown making these noises, despite being literally giant dolphins (they squeak, click or whistle).
Forgetting only baleen whales eat krill and plankton. Their baleens are specialized for sifting through water for small animals. If the whale has teeth, it's built for gripping flesh and eating larger animals. Sperm whales are often victims of this error, despite being famed for preying on giant squids.
Portraying the orca as a lesser predator than the great white shark. In real life, orcas are bigger and stronger than great white sharks and the only predator (besides humans and other great whites) that preys on them, with every account of an orca attack on the shark resulting in the entire shark population evacuating the area, including feeding grounds. Even a large great white knows better than to attack an orca, which have a method of killing sharks by flipping them upside-down and hold onto them until they drown. This is more of a case of Science Marches On, as recorded orca attacks on great whites are fairly recent.
Mollusks and Crustaceans
Cartoon decapods and cephalopods both seem to suffer from the Four-Legged Insect trope. Why? Because less legs/tentacles means it takes less time to animate!
Decapods are often given six or eight legs instead of the 10 legs that they have in the real word.
Though most people know that octopuses have eight tentacles, animated octopuses often are drawn with six tentacles. In rare instances, squids might be depicted with four tentacles as well.
The Removable Shell trope applies to this as well with examples where hermit crabs and mollusks are able to shuck off their shell and put it back on at whim.
Similar to the above example of writers confusing salt and freshwater with each other, writers may also confuse sea snails and freshwater snails with each other as well. Watch out freshwater snails, that salt water will kill you!
Octopuses almost always will have faces — complete with Cartoony Eyes — placed on their huge foreheads. As a result, what appears to be a real octopus' mouth becomes a cartoon octopus' anus!
The same happens to squids, but to a lesser extent, since they normally keep their eyes where they originally are (near the tentacles).
Cartoon octopuses and squids often have a siphon on each side of their heads as if they are ears. Real cephalopods only have a single siphon. In Japanese media they will have their siphon placed underneath their eyes as if it was some sort of tube-like mouth.
The Giant Squid being portrayed as a Sea Monster devouring humans. When encountered alive by humans, giant squids consistently had no intention of attacking them, preferring to either silently observe them or simply ignore their presence. Any reliable aggressive accounts may have been due to the squid acting out of self-defense.
Bivalve shells almost always function as mouths, rather than sides. Sometimes, said shell "mouths" have tongues inside of them or even having their soft bodies posing as tongues!
On that note, bivalves like to trap people with said "mouths", even though real-life clams are relatively harmless creatures.
Barnacles are often depicted as parasites that cause harm to whales. In reality, barnacles rarely cause harm to the whale unless there is a large amount around the whale's blowhole that prevents it from breathing.
Though not a marine crustacean, pillbugs (order Isopoda) are almost never referred to as crustaceans; most people would mistake them for Myriapods (centipedes and millipedes).
Echinoderms
Though starfish are faceless critters, they are usually animated with faces (eyes and a mouth) on their backs.
Sea urchins seem to only exist to hurt people with their spines. This is not true, though. Urchins actually help the environment; they are like vacuum cleaners of the ocean floor. They help to clean up any kelp that has fallen to the sea floor, maintaining a healthy kelp forest ecosystem.
Cnidarians
A jellyfish's sting is treated as electrical rather than venomous.
Corals seem to equal plants and grow everywhere. Despite resembling plants, corals are animals part of the Cnidaria order, and the types typically seen only grow in warm, shallow water.
Sometimes, sea anemone are depicted as plants as well. However, the relationship between sea anemone and clownfish is often brought up in fiction, showing that the writers have Shown Their Work.
Leeches are usually drawn with a Lamprey Mouth, even though real leeches have a much less pronounced mouth. Alternatively, they may have a tube-like mouth instead.
and freshwater snails