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
These devil water scorpions turn whale poop into black pearls. They cant leave the ocean floor so Get you a pike and stimulant. Stab these from a distance. Put the stim in the hot bar so if you get pinched you wont wall asleep on the ocean floor
Great source of oil and black pearls, but will knock u out in 1 to 2 hits instantly. So keep at distance, use sarcos or megaladons to kill. Also will follow u out of water, so flyers can gang up. Very fast swimmer so be careful.