When entering the water, go under and look around for a medium sized fish lunging for you. If there is, swim back to shore asap. The piranha should be near the edge of the water. Pumch it without getting too deep or close to it. You can easily gather fish meat this way.
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