Because he can already give people seizures by taking to the part of their brain that used to be a fish, and has superstrength and superresiliance; hydrokinesis on that level would make him TOO awesome.
With the water hand, Aquaman can instantly dehydrate anyone he touches, killing them immediately. Is that not badass enough for you? Or would you like more examples of his awesomeness?
In one JLA story, Aquaman faces off against a villain who openly states that Aquaman's powers are worthless on land. Aquaman responds by finding a specific part of the villain's brain which evolved from aquatic ancestors, and gives him a concussion. Which is pretty cool...until you realize that the guy Orin was fighting was a space alien. How could Aquaman even know that the villain's species evolved from the ocean, let alone have enough knowledge of his physiology to pinpoint a specific part of his brain?
If you reread the scene, you can see from both the art and dialogue that Aquaman was telepathically scanning the White Martian's mind as he was busy insulting him. That's how Arthur found out about the said weakness.
This was a martian if I recall correctly, presumbably Aquaman has had ample opportunity over the years to telepathically study the Martian Manhunter's brain and find the spot in question.
There was absolutely no guarantee that the Whites and the Greens had the same weaknesses, and pulling out a weapon you don't know is going to work is pretty much suicidal.
Whites and Greens are basically the same species, biologically speaking; their differences are cultural, exacerbated by shapeshifting ability. If you assume that their nervous system remains unchanged by that shapeshifting into a different preferred form, the structure of the brains should be the same.
It doesn't even matter if they're the same species or not, as products of the same chain of evolutionary development there's going to be analogs of brains structures in a whites head that Aquaman has found in John's, especially something so sncient ir evolved be their ancestors emerged from the sea. Also the fact that Aquaman was pretty much outclassed on every level except the telepathic one means it makes perfect sense for him to to use to use the one weapon he had that was at least on par with a Martian's own abilities, whether or not he knew for sure it would work since. When it's a life or death situation you try anything that you think has a chance of success.
The guy clearly didn't know what he was talking about, and Aquaman's attack was therefore just ironic or convenient. Arthur's superhuman strength and speed and his telepathy can be extremely useful on land.
His strength and speed would be pretty much useless against a white Martian, who are at Superman levels in both areas.
Perhaps the White Martian had shape changed all the way down to his brain 'chemistry', thus he gave himself the reptillian aspect of the mammal brain that Aquaman then proceeded to fry.
Actually, he does use telepathy fairly regularly. It just gets forgotten in the rush. My own opinion is that the main thing restricting his telepathic potency is lack of practice and training. Hence he mostly uses it either within its specialty ( aquatic life ), which comes easy, or when he's pant-wettingly desperate ( evil Martians, Millenium Giants, etc ).
I always assumed it was something that worked on Martians but not humans. (Yes, humans evolved from aquatic life, but it's along a completely different evolutionary path. There's no reason the portions of our minds that come from aquatic ancestors couldn't be too small or unimportant for him to affect, or undergone so much change over the ages that they no longer count.) After all, he never uses that ability on anyone else, even when it was the same writer on the title.
I figured it was because giving somebody a seizure, as he did (not a concussion), is a pretty brutal attack, and Arthur's one of the good guys. It's not something to trot out for just any old villain, but a hyperspeed Martian is a valid target for bringing out "the big guns".
During the infamous Detroit League, Aquaman used telepathy to give a mock-seizure to the complaining Steel II, and some days later to Vixen, to prevent her from disobeying a direct order. J'onn J'onzz himself, the usually jovial, quiet, philosphical Martian Manhunter, grabbed him by the collar berating him in front of the Detroit Leaguers and the Classic League. Zatanna herself, shocked and bewildered, after being told from Steel II of his earlier attempt, stammers something about Aquaman having never used his telepathic power on a human being before, noting that the League was descending to madness. We can safely assume that human telepathy is a big no-no in the heroic community, a violation of the individual free will too blatant to be used if not in the direst emergencies.
Compare with Miss Martian, a pacifistic White Martian who, as a part of her pacifistic vows, swore to refrain from even mind-reading a human being without his explicit consent.
Orange shirt, green pants. That is all.
Mermaid mimicry. Green tail, flesh/orange-ish top.
The Legend of Aquaman Special shows his costume as the standard Atlantean prisoner uniform that he picked up while doing hard labor◊. Vulko asks him several years later why he's still wearing it, and his response is that it's meant to be ironic.
Also, on Smallville they implied that he chose the colors because, at least in that incarnation of the character, he was attending college at the University of Miami, where the school colors are orange and green.
Also, during the 2011 he describes his skirt as chain mail. So, it can be easily some sort of utilitarian Atlantean chain mail armor, used in prisons or for infantry troops that Aquaman used for added protection.
How is it that Aquaman's telepathy only works on marine life? It can't be because of the relative simplicity of their brains—a whale has a far more complex brain than a minnow, but he commands them both with equal ease. Why can he command sea mammals but only stun land mammals? If it were explicitly magical in nature I could accept A Wizard Did It, but like most Psychic Powers in comics, it's played as semi-scientific.
Seing as how Atlanteans are a somewhat magical people, it probably has more to do with magic and spirituality. My guess is that creatures with a stronger spiritual connection to water are easier to command.
So, out of curiosity, what's his status over amphibians?
Little to none. Even those that are aquatic are limited to land-based water bodies, after all. Maybe some small to moderate measure of influence for the more aquatic species and life stages, tiny amount of influence for the truly amphibious, and none at all with toads.
False premise. Aquaman's telepathy *doesn't* only work on marine life. He's been using it on all kinds of things since at least the 60s ( humans, Kryptonians, Martians, dragons, weird disembodied computer things, undead, Despero. . . ). Its just more powerful on marine life, presumably for mystical reasons.
The Aquaman 2011 series puts a dampener on Aquaman's telepathy, highlighting how the average marine life has a simpler brain structurenote "Dolphin's are a different story.", and Aquaman's telepathy, rather than the complex manipulation of the thoughtstream exibited by the Martian Manhunter and Saturn Girl, is more akin to the simple manipulation of low-level functions that's part of Princess Projectra's powers (for example). Thus, while Aquaman can essentially give a seizure to everyone, find easier controlling simple mindsets by melding urges and compulsions to their brain activity.
A few issues in, Aquaman commands some angler fish to provide light for him and Mera. As they near their destination (the location of incredibly dangerous aquatic humanoids) the fish swim away. When questioned by Mera, Aquaman says that their survival instinct was overriding his telepathy.