There were no conditions placed on the sign, it literally reads "NO SWIMMING". It doesn't say "warning", "beware", "alert", "at your own risk", it says NO SWIMMING. That's not a warning it's explicit, it means don't do it, AT ALL.
Completely different guy but do you know what killed the guy that put the mortar tube on his head? When the mortar is launched there is an exorbitant amount of recoil that occurs to launch those mortars upwards of 200 feet in the air.