The first time I used one was a few years back when I was on vacation in Cancun. I put all my credit cards, my room key card, some paper money in an Otterbox when I was on the beach or snorkling. Everything was kept dry. The first time I put an electronic gadget (an iPod) in an Otterbox, I was a little worried. So I put my iPod in a leather fit case just in case it gets a bit wet, then lock it in an Otterbox and dip it in the pool. The Otterbox kept everything dry again. I've learned now that I shold trust these waterproof boxes.
They are a bit bulky especially if you get the Armor series which are really designed for field users. They need more shock, bump, scratch protection than just waterproof. For your vacation, pool-side use, the thinner ones should do just find.