Hi Marylu; not knowing anything about you I wouldn't presume to suggest to you, "do this", or "don't do that".
My wife is doing the same trip on Golden Princess on Oct. 6 and we have just spent a lot of time with the shore excursion brochure; and the choices are quite varied. But everything in the brochure is there because it appeals to someone.
What we do is go through a process of elimination. We read the tour description, say yes or no, and move on. Little by little we narrow it down to what really interests us. It just kills me that I'm spending all the time I am helping her and our family friend plan the trip--and I'm not going.
There are good snorkel tours in all three ports, but I should mention that if you are use to snokeling in Hawaii or the Caribbean, you might find Mexico's West Coast a little dissapointing. It's still fun, but the water clarity and variety of sea life is just not as good.