As a SCUBA Diver and Divemaster, I'll address your first question, Sharks. Yes, there are sharks living in all oceans, but the locals all know where to take you to avoid the vast majority of problems. They will take you to areas... such as those that are protected by reefs, etc. I've been on hundreds of dives and snorkeling and have never had a problem. You are far more likely to experience thinks like jelly-fish stings, etc. than shark encounters. Listen to the tour (dive) operator. They want you to have fun and not end up with problems.

Regarding what to do with personal belongings, take ashore only what you need such as your ship card, etc. DONT take things like expensive jewelry, large sums of cash, multiple credit cards, watches, etc.

Any reputable tour operator will tell you the same thing "that their reputation is far more valuable to them than what you leave on the boat while you're snorkeling"

So, take ONLY what you need, then leave it on the boat that takes you out snorkeling. They usually have storage areas for just such things. I always make it a point to protect things that have personal information, such as charge cards, etc.

Finally, if you are still worried, there is a lot you can take in the water with you in a mesh dive bag, such as an underwater camera, your ship sailing card (it's like a credit card), real currency (its actually made of cloth, not paper), etc.

On the countless snorkeling tours I've been on, I've never had a problem... not saying you couldn't, but protect yourself as best you can, and enjoy the experience.