Corpse size has a lot to do with it. I wouldn't swim in even a large pool with a dead human in it (knowingly), but one dead fish or rodent or dozens of dead tadpoles or bugs? Not an issue.
Heck, most household swimming pools have dozens of dead bodies in them, but they're 99% insects.
The whole premise of this meme is a bit silly. If there was a corpse floating near the beach, I think most people might wait for the corpse to be removed, and perhaps even a reasonable cause of death to be determined, before entering the local area. The same is true for pools.
Fingernails aren't flesh; there's no such thing as a dead fingernail because there's no such thing as a live fingernail. It's like shed hair floating in the pool; kinda gross, sure, but not dead body gross