You would do much better setting this up yourself instead of going on the cruise ship excursions. They are quite expensive and are very crowded.
The only upside is that you get off the ship earlier than the other guests because of the prepaid excursion. We have taken many tourists from the cruise ships that either book ahead or just come to the island on their own. You would have plenty of time to come to San Pedro, shop, lunch on your own, not with over 70 other cruise ship passengers, and get to snorkel at 2 spots versus one.The cruise ship trip to San Pedro only includes Shark Ray Alley not Hol Chan Marine Reserve which shouldn't be missed. San Pedro is a 15 minute flight from Belize City and runs around $50.00US round trip. If you would like you could email me for more information.
Dawn O'Connor www.barefootwatersports.net