I recommend Alfonso Graniel "L'il Alfonse" for snorkeling Hol Chan and Shark Ray Alley. He is extremely knowledgable and tons of fun! You may have to take a water taxi into town to meet up with him? It's a ha;f day tour.
I also loved the catamaran sail to Caye Culker with SEAduced. Very relaxing. 2 snorkel stops (same spots as above). Full day. They have snacks, rum punch, Belikins on board. You have lunch in Caye Caulker and about an hour after to look around. You can swim at the Split. Then sail back. Lovely trip!

Lagniappe Provisioning with Simon. Worth it! Everything is fresh and you don't have to shop.