Yeah, yeah, everybody's done this one.
But I felt like redrawing one of the gems I had to make up on the fly for my Birthstone Gems
in my own style. I really like how the lighting and colours came out on this, and that sunrise? Yeah, I didn't just grab a gradient from somewhere. That was done by me. In Paint.
Anyway. Aquamarines, in the headcanon I made up while drawing this, are (of course) ocean gems--how can you NOT, when their name literally means "water" and "ocean". But unlike Lapises, who use their manipulation of water to terraform (and fight), Aquamarines are DEEP underwater gems, and rarely see the surface. They build and maintain the resource extraction facilities on the ocean floor--because not all the "good stuff" of a planet's crust is under DRY land! (I mean, come on. Do you honestly think Homeworld could see just HOW much of the Earth's surface is water and _not_ figure out some way to exploit that part too? Same for other planets with oceans.)
As such, they tend to be compactly built (5'-5'6" usually) and fairly muscular, and while Gems never need to breathe anyway, Aquamarines are specially built to withstand the pressures under the ocean. (And see in the dark--which I'm not sure is a standard Gem ability or not.) Specifically, they are programmed to _move_ with the amount of force needed to push against the ocean pressure as if it was just normal air to them, so they don't slow down in their work. The downside of this is, on the rare occasions they DO come to the surface, they tend to make way too big of gestures and trip over things because of taking way too big steps--it takes them a while to mentally...calibrate themselves to the surface.
This MEANS, that this ocean goddess, rising elegantly from the waves like unto Botticelli's Venus...
...is actually a big ol' dork who might whack you in the face by accident while talking.
Aqumarines aren't made in bulk as their jobs usually only need one, full-time, worker in a fairly large area, and said work rarely requires them to go on land, so they tend to come across as "that weird old sailor dude" when they _do_ come to the surface. It gets BORING down there, so they tend to take up odd obscure hobbies, decorate themselves with bits of whatever they can find in sunken/abandoned man-made facilities (shipwrecks and old oil rigs, etc.)--and while their normal outfits tend to be either full-body and streamlined or minimalist, both to avoid drag (so...basically diving suits or bikinis), they also sometimes copy the fashions of the only other gems they usually come into contact with--Pearls.
Yes, it makes sense that Pearls would be made underwater, and...wait...did I just make up underwater kindergartens? (As probably many other fans did before me, let's be honest?) Huh. Well why not? like I said, a LOT of the Earth's crust is underwater and you wouldn't want to waste all that part. The raw materials for making Pearls _are_ plentiful down there, as, after all, that's where normal pearls come from. Our Aqumarine here is stationed on Earth, so she wouldn't've made _our_ Pearl--and none were made on Earth at all, as that's a luxury "item" you start producing AFTER a colony is good and established--which of course never quite happened.
Speaking of, during the Gem war, no Aquamarines actively joined either side...but those who agreed with the Crystal Gems' cause (once they found out about it) kind of _quietly_ showed their sympathy towards the native Earth creatures by rescuing them whenever they saw one drowning within range. This gets them the nickname of "Lifeguard Gems" by some...but since Aquamarines often change their form to a fishtail from the waist down so they can move about faster in water without losing the use of their hands, the primitive people they were rescuing had a different name:
ABILITIES: Well, the under-pressure-movement and dark sight stuff, also many Aquamarines have either markings for underwater camoflage, "bio"luminescence spots that they can turn on and off at will, and of course they can shapeshift . BUT, since one can only change into something one is familiar with, they really specialise only in sea creatures. Put this chick here on land, tell her to copy a horse and she'll totally screw it up.
And that's my overly-long explanation of something that's a fan character so you probably don't even care anyway, as you have your OWN fan interpretations of things. If you _did_ read this far, I salute you!