These things are the worst. As soon as your foot hits the water, it is like you have a tractor beam on your feet for them to zoom straight towards you, swarm you, and then proceed to kill you. If you’re new to the game, and you see it coming, I have one word of advice: SWIM.
These things will spawn in your water pen and kill your tames if they're on passive. Make sure you have Dino's to kill them or foundations all around your pen, they've killed so many of my Icthyosaurus' it's not even funny.
I'm around lvl 30 and ever since I started the game I can honestly say "I hate these creatures!" I've died by them more then anything else lost two parasaurs, a raptor, and who knows what else. Advise keep glancing under water to see if any are around, avoide rivers crossing on foot. I like rock hopping and dino swimming they can take more damage