Mazatlan is really not a big loss. You would really need to have a tour lined up for it since it really is a small port town with a cruise terminal type shopping area built for the cruise ships. There are some restaurants and such outside of the terminal but not much to see. We went horseback riding on the beach which was fun but I bet those tour operators are in financial pain right now with no one stopping...
It might be cold water in December Penny.... I'd bring your wet suit top just in case. If we had not been snorkeling in PV... we might have tried it there, but Cabo looked like it had better snorkeling choices...
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007