Shallow pool
I really hope that these signs are leftover from some sort of office joke, or a prank that has long since stopped being funny. I can think of no other reason why someone would feel the need to place “no diving!” signs on a toilet.
Unless there was some kind of delirium involved, or alcohol, or both, the only person small enough to go diving in one of these wouldn’t be a person at all, but a guinea pig. I really hope it wasn’t a guinea pig.