They are found at the bottom of the ocean (island and ragnarok, not sure about the center), they are great for farming black pearls as they yield a few per, however, make sure you fight them on a water Dino that has a lot of torpor and avoid it’s hits, as it deals pretty powerful torpor to the player and their mount.
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.