These aren't limited to under the water, and can float over rocks. If they manage to melee you, you're probably going to be knocked out by their tranq and slowly killed. They're easy to kill at range if you're smart, and very much worth it for the chitin and silica, so just be careful to either have a tamed Dino nearby or to stay at range
If you need black pearls on valguero, there is a cave hidden in the jungle on a hill near green ob that you can enter and drop down a big hole into a water pool. There are 4 or 5 pools in the cave, each with up to 15 eurypterids in them, you just have to kill the eels before you get the eurypterids. And the beauty of this cave is that if you leave it (make sure to bring grapples to get out if you don’t want to go the long way) the eurypterids will be back and you can farm another few hundred black pearls. It’s even easier in single player where you can exit to main menu to refresh the spawns.