The addition of the fine is what causes this sign to be funny. Once you are dead, charging you a fine seems unnecessary. JOKE EXPLAINED.
What's funny about this is that it forces you ti imagine the type of people that would keep their money in their underwear. JOKE EXPLAINED.
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.
This sign is an 'up punch' toward the self superior people who wear sunglasses indoors. In doing this, it is using those people's sense of self superiority to frame them as inferior. JOKE EXPLAINED.