I’ve had this fear, and it still resurfaces from time to time. And it is not just in lakes, oceans, etc, but not mainly in showers. I thought I was crazy, but for some reason I would panic whenever I had to get in the shower because all I could think about were sharks and fish and I somehow came to the conclusion that they would be in the shower when I hit the water. It also happened even when I wasn’t near water. I think it may have something to do with ichthyophobia or hydrophobia. It’s actually reassuring that someone else had similar thoughts (though I am not happy that you are going through this). Everyone I ever worked up the courage to tell (even therapists) were really weirded out and confused. But it’s just a weird ocd/anxiety thing. You’re not alone :)