I have been to Mazatlan many times and will be flying there in 2 weeks. (Dad has a timeshare for New Years) Don't know about the snorkeling there but will check it out while I'm there. We did an excursion to Stone Island once, rememer arriving but not leaving. There are small beach restuarants and banana boat rides, plus horse back riding.
Carnival Splendor (3 days of towing back to port)