If you were able to create your own creature within AoC, what would it be?

Nice try Intrepid Spy but no really.

A giant octopus that can javeline you off the ship with a tentacle and pull you into the watery depths to drown while paralyzing and poisoning you is pretty high up there.

An angry storm cloud that can change shapes to use different class abilities on top of its lightning based magic. Ignores hard cc unless it's solidified for an ability.

I know this isn't what you meant but it technically tics all the boxes.

After thinking on the prompt a little more I think it would also be cool for a few other sea creatures.

Now we do not have any confirmation on gathering at sea per say, but entertain an assumption that there might be for a few of these ideas.

If there are kelp forests with ample fishing clams and pearls, it would be cool for there to be eel grass. Eel Grass has long slender bodies that look like kelp, tails held up swaying with the current, their heads swiveling near the sea floor looking for prey. Their camouflage makes them a shocking surprise for divers if they bumble into their line of sight. Underwater rogues have a nasty bite

Out in open water there is always an inevitable pile of debris clustered together due to the currents. Flotsam and jetsam get tangled up in seaweed, kelp, and algae. Sometimes this can be a blessing for sailors as the salvage can help repair their ships or prove a fertile fishing ground. In Verra glowing yellow round slugs feed on the debris on top of it. However a predator has learned to mimic them. Think a basking shark meets angler fish. Go near it and suddenly you find your self and boat pulled in with its large powerful jaws. If you try to fight it, it has a powerful razor like tail that it can go link on your weed lookin butt and try to chop everything around it.

Risk and reward.