If you get hit once by this creature early to mid game, your practically dead unless you have high level dinos nearby to kill it. It's got a decent move speed on land as well. Best way to kill them is to aggro then them and swim up to make you sit below you. They cant swim upwards so you know.
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.