What's a good way to keep track of the time at a water park?

I want to make sure I reapply sunscreen every 1-2 hours, but I don't want to carry my phone or wear a watch on my wrist. I'm thinking it would be ideal to have some kind of super light waterproof pocket watch that I can attach to my swim trunks or something. But I don't know if such a thing exists. How do you guys normally check the time when you're at a water park?