This Map shows high quality coverage over most of the island of Cozumel (the part with real roads, at least) on GSM 1900 (the same band as you use in the US). But it's $1.49 per minute, so factor that into your plans.
Telefonica Moviles Mexico serves Cozumel. Telcel Mexico also has a roaming agreement with T-Mobile but it looks like they don't have coverage over the majority of the island.