I can only comment on the Mexico Rocks snorkel trip and Caye Caulker
We've had some great mornings at Mexico Rocks and Tres Cocos with both 'lil Alphonse and Searious as guides. The best trip was with a guide from Searious, named Jeffery. At our first stop he did the typical guided tour of the area. On the next stop, we did a "drift" snorkel, where Jeffery let us explore on our own, swimming with the current parallel with the reef. He stayed in the boat and kept us in sight. That way we didn't have to do the circular tour. If you're comfortable enough in the water to kind of be on your own, it's a great time.
Now for Caye Caulker - why go all the way to Caulker to go to Hol Chan/Shark Ray? You can book those trips from San Pedro. We took the water taxi to Caulker and then wandered around to find a snorkel trip to go on. We selected "Johnny's Tours" and had a great time. Two stops and beautiful snorkeling. The second stop, however, was called Coral Gardens and the water was much deeper - say 15 to 20 feet. It is best enjoyed free diving down to the coral heads, so if that's not your style, it's maybe not your trip.
San Pedro is a magical place - not fancy, totally laid back and just about heaven. It only takes one time to become an AC-aholic.