Cruise to Nassau and Princess Cays, The Bahamas

We went on a cruise a few years ago, and forwent buying excursions from the cruise ship. We were able to get better rates, and better excursions from locals. However, this trip was to Mexico. I've been "warned" that The Bahamas is "more dangerous" than Mexico, and to only buy excursions from the cruise line. We would like to do very basic excursions, such as snorkeling, or renting a scooter, or any adventure-based excursion. Has anyone bought excursions from individuals on the Nassau and Princess Cays islands? How was your success? Do you remember the rates compared to your cruise? We are traveling with Carnival Cruise.