What's funny about this sign is that there is a perceived extreme threat (the shark) that is given as a reason to do something small and annoying (peeing in the pool). JOKE EXPLAINED.
The 'sorry' addition is what is funny about this sign. Signs are designed to state information clearly and professionally and the 'sorry' gives the sign a human element that is uncharacteristic. JOKE EXPLAINED.
This sign is less funny and more a statement about how much the sign owner values sarcasm. JOKE EXPLAINED?