Did a little snorkeling, and it's definitely done there. Now it's been years since I've been there, so I don't know if this still applies, but although it had a higher tourist focus than some of the other places we'd been (like Chiapas, Tehuantepec), it had a much more laid-back European feel to it. Nothing at all gaudy, no hype or neon lights. It felt like Mexico-meets-Europe, whereas Cancun feels like you just left Mexico and re-entered the US by comparison.

i'm a snorkel fanatic. on the principle that where there is good diving, there is usually also excellent snorkeling...here are some suggestions:

about 40 minute south of cancun is a place called playa del carmen. the snorkeling around there was good, not the most amazing i've seen but very really nice. at the time PDC was an incredibly low key, beautiful and non-touristy place (this was in '01). i hear it's changed a lot since then and has become almost indistinguishable from cancun, home of the frat boy.

cozumel, an island to the southeast of the coastal PDC, is known for it's diving.

i've also heard, but not seen for myself, that the reefs in xpu-ha (a little further south from PDC) are astounding.

btw, if you fly into cancun and head to tulum (both on the east coast) you'll pass by all these places above. chichen itza is central, not on the coast.

if you're up for traveling a little more adverturously, keep traveling the east coastline until you hit honduras, specifically the islas de la bahia which are a little north of the mainland. three islands: roatan, utila, and guanaja. utila in particular has a rep for being low-budget friendly. the diving is phenomenal in these islands...the belize coral reefs extend all the way to these islands and they are incredible.

good luck!