There is a lovely little spot in the Hol Chan Marine Reserve where the weather is warm, the waters are crystal clear, and the diving and snorkeling is unparalleled. That is, if you don't mind getting in the water with a hoard of sharks and rays, which you should.
Known fittingly as Shark Ray Alley, this popular dive spot is a hotspot for largely harmless nurse sharks and smaller rays. The spot came to be not because of any feature of the environment, but because local fishermen used to come to the spot to clean their catches, filling the waters with morsels that were perfect for the bottom feeding nurse sharks and rays. Soon the animals recognized that they could come to the spot for a reliable meal and returned in droves. Then, in the perfect circle of life, the fishermen got the local dive masters to start coming to site, letting divers get up close and personal with the animals, and actively feeding them to keep them coming back.
Nowadays the sharks and rays can actually hear the boats coming and swarm the area when their gravy trains approach. Divers and snorkelers get to jump into the teeming waters to swim alongside the animals, while dive-masters keep them in the area by feeding them. The four-five foot sharks are not really interested in making a meal out of any tourists, but they are sharks nonetheless, so a diving in the area is probably best reserved for those who are not afraid of the animals.
The water is only around eight feet deep, and there are a number of beautiful reefs to check out while you are beneath the waves, but again, there are a ton of sharks and rays just swimming around. A great place if that's your thing.