Usually I snorkel in Florida mostly, but my favorite places there are still in recovery from hurricanes. (yeah, and Katrina is on the way this weekend...)
I did the Shark Ray Alley and Hol Chan Reserve trip twice with Alfonso Graniel, a terrific guide. He knows every creature on the reef and is an animated and energetic fun guy. He is careful to pace for the group if there are older folks or children. The Alley had bigger creatures (nurse sharks, sting rays, groupers, blue tangs, parrotfish. Hol Chan has a huge mix of fish, morays, sea cucmbers, urchins. Alphonso also took us on a detour to see loggerhead turtles where the conch fisherman harvest their catch. If there's time, you might get to retrieve a shell when they're done. (You can take 1 per person cuz it's outside the Reserve boundaries.) He has a nice webpage if you google his name.
On the Mexico Rocks trip we did a drift snorkel. The fish were smaller and more timid types. Lots of sergeant-majors and I just loved the Rock Beauty. We saw lobster there, too. I can't remember the two guides names. It was a quieter trip. Still lovely.
Lots of the shops have field guides to identify fish and corals. I enjoyed having that to show folks when I arrived back home.
Each trip has it's own special qualities. Definitely do SRA and HC. Add on other trips for more fun. You are gonna have a blast!! I still have vivid dreams about being in the crystal clear waters there, with so much to see!