Water Proof Pocket in your Swim Trunks

Today we are headed to the local water park.  They don't allow outside food or drink which means I'll need to carry my wallet so we can purchase overpriced pizza for lunch.  Problem is I have a leather wallet and I can't be splashing around with cash, frequent eater cards from the bagel shop and my library card in my pocket without expecting major water damage.  Not to mention my car key fob.  The answer is a water proof pocket in my swim trunks.  The answer today is a zip lock bag.