Xanadu is a truly wonderful place to stay and Berlize has a lot more places to visit than many other Carribean destinations. The Mayan Temples are fantastic, the zoo is one of the best I have seen, the zip line has 9 segments to whiz through the trees and the cave tubing is an unusual treat. Xanadu is very good at sorting out trips to see the sights and/or for snorkelling.
You can snorkel off the pier at Xanadu, but there is not a reef you can wade to off Ambergris Caye.
The snorkelling trips are pretty good though.
If you want a place to snorkel directly off the beach, Provodenciales in Turks and Caicos is probably better, although as a "dry" island, the vegetation is very different from the rainforest lushness of Belize although beautiful in its own way.
Or, there is always Na'am Bay near Sharm al Sheik on the Red Sea in Egypt.
For a really unusual snorkelling treat, come to Belize in the 10 days around the full moon in April, May or June and snorkel with whale sharks.