My wife and I went to Puerto Vallarta during the week of February 28, 2004 to March 6, 2004. One of the days we went on the Vallarta Adventures snorkeling and whale watching trip to the Marietas Islands. Unfortunately we went too late in the season and only saw two whales. The snorkleing was merely OK; I was disappointed after snorkeling in Maui last winter. If you are going just for the snorkeling you will probably be disappointed.
However, the rest of the trip was a lot of fun. The crew was great, the food was pretty good and the open bar was open the whole trip. The crew leader was Chuby Do and he was like a Mexican Adam Sandler. The crew really busted their butts to make sure everyone had a great time and provided great service. The trip the secluded beach was great, and it was nice to be at a beach with no vendors. The only problem was it was very hot and there was no shade to speak of. As you can tell I am from Minnesota and not used to that much sun in the winter.
Make sure you catch the crew's rendition of Summer Lovin' from the Grease Soundtrack on the way back to the port. It may have been the funniest part of the trip.
If you are looking for whales go earlier in the season and if you are only going for the snorkeling you will be disappointed, but if you are going for a fun time with a great crew,this is right up your alley.