I recently cruised to Tahiti on the Radisson Seven Sea's Paul Gaugiun and was wondering the same thing. Check with the cruise line and see if a snorkeling adventure is included at one of the stops. Otherwise, check out the islands and see if there are any beaches that are avaiable to swim at. Be SURE that it is safe, you don't want to be carried away by an underwater current. I met a couple on my cruise who rented bikes in Bora Bora and took along their snorkeling equiptment and stopped at several beaches for some relaxed snorkeling. All of the natives are extremely friendly and would be more than happy to help you out.