You can pick up some neat little waterproof, plastic things that hold your cruise card, a couple of bucks. They ususally sell them in the ships shops and they are great for going on excersions. As for towels, etc. if snorkeling on a boat, they will be safe while you are snorkeling. Some beach provide lockers, which you can also use to put your stuff in. I never bring lots of cash, just enough for a bit of shopping, tips, etc. Also keep your passport, drivers lic in the safe in your cabin, but have heard its always a good idea to travel with a copy of passport while off the ship, will have to do that myself.
On snorkeling excursions u can normally leave items like towels, cameras, etc. on the boat that takes u to the snorkeling area.
I've seen the little plastic watertight containers Donna spoke of, but
I use a water proof fanny pack for cash, id, credit cards and such. They can be purchased pretty cheaply ( $10- 15) at most luggage specialty or travel accessory shops.
And the final lesson is ... never take anything too valuable with you on the excursion. I think our RoboCop is the expert on that topic. He recommends taking 1 credit card and leaving the other one in the room. Limited amounts of cash, etc. I think he likes money belts, too.
What Mrs Robo and I do is take as little with us as possible. When we go somewhere to do something that we know we will be getting wet we bring one of those disposible waterproof cameras and for the stuff like jewelry, ID, and small amount of cash and CC, we place those in one of those waterproof containers and either carry it with us, or more likely at the beach we actually bury it beneath the towel or beach bag in the sand. The container keeps the sand out and burying it helps keep people honest. Just a few inches below the sand is fine, no need to bring in a steamshovel! <G>