Might I recommend a "water wallet"? If you are nt familiar with them, they are approx. 5x7 and can be worn on a belt, around your neck, cliiped on your hip, etc.
They are used to hold money, cc's, room keys, cigarettes, etc while enjoying water rides and swimming, boating, etc. We purchased these a few trips back and have used them frequently locally as well as on vacation. I think we paid $10/ea for them. You can do a google search for "water wallet" to find locations/pricing to purchase if you are interested.