Best way I’ve seen to hunt these is to swim a short way into the water and peg it with a crossbow bolt, that way it Aggros to you and swims to shallow water where you can finish it off and collect those black pearls, be careful though, it doesn’t do a lot of damage but it will knock you out in one hit, and can follow you onto land.
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.