In the Ocean, I would hate to have an encounter with Jelly fish and Sharks, but I also hate Crabs and try to thread water rather than walk. I hate to get nipped by them.
I don't swim in lakes very often. The stories of brain eating parasites has terrified me. I know they are found in warmer water, but still that is down right scary. Yikes!
This is the reason I really don't like walking over the beach or swimming in the ocean. The idea of there being creatures without me knowing it is horrifying. Last time I was there, I saw a couple of jelly fish, and I'd rather not run into one of those. I'm really scared of crabs, so that would probably be the worst one for me