They are, quite literally, Sea Scorpions; so unless you have an aquatic mount with some hefty torpor resistance, stay away from the sea floor. They can't swim, but hover right above the ground, kinda like a trilobite (They spawn in Viking Bay on Ragnarok, and can easily follow you onto land from there). If you really feel like it, they do drop Black Pearls when harvested, and so can be valuable as long as you keep an eye on your depth/what else may lurk in the darkness.
Be very careful about these-they do tons of torpor damage, I was down on the bottom of the ocean (on the island) and I saw a pack of these sea scorpions. I decided to dismount off of my mosa and kill them with my pick, as that would get more black pearls. When I got close to them one of them-a level ten hit me and knocked me out in one shot. I was level 88, wearing journeyman flak and apprentice riot. And they knocked me out in one hit. The moral of the story is that these things are very dangerous.