You can pick up a waterproof wallet at K-mart, Walmart or Magellans.com.
It will fit around your neck or around your waist and tuck into into your trunks or fit into a zippered pocket of your bathing suit. They work great. You can also go inexpensive and use a zip-loc bag for your credit card, ship card and cash. Leave the rest of the stuff on the boat or shore.
These things still work just fine if they happen to get wet.
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hunting/fishing license lanyard - hangs around your neck ...
and is waterproof. This was suggested in the 'Practical Advice' section under 'cruise packing'. Sounds like a great idea and probably inexpensive from any hunting/fishing store. http://nationalleash.com/money_tuck.htm
If you are doing a snorkeling excursion, you can leave your stuff and your ID on the boat while you snorkel too, should be fine. Only bring along what you need for the day, leave everything else back onboard in your safe.