Ziploc freezer bags. Or for that size, I use some small pill storage bags from Walgreens. I keep one Altoids tin sealed with a wrap of electrical tape. That's not completely water-tight, but it would survive a quick dunk. It also makes it harder to raid the contents if I don't really need to.

The Witz and other brand wallet-sized waterproof boxes are available at any boat equipment store. Oddly enough, Disneyworld resorts sell them in their gift shops and pool-side dining areas. It's like they knew you were coming.