Breathing Restricted UnderWater

This sign is quite rhetorical because obviously who breathes underwater unless the person is a mermaid. And it seems like the owner belives that maybe his/her pool might be a famous vacay point of mermaids and they must be hanging around often. Hence restricted them from breathing underwater. But here, we are left wondering what harm might it cause if they breathe under the water unless they pee from their mouth or nose!