Cozumel is a larger island south of Cancún, best known internationally as one of the world's top scuba diving destinations thanks to the Palancar and Colombia reef systems just offshore.
Getting there from Cancún
Cozumel isn't a direct ferry from Cancún itself — the passenger ferry runs from Playa del Carmen, roughly an hour south of Cancún by road, with the crossing taking about 45 minutes. Day trips from Cancún are possible but involve real travel time; an overnight in Playa del Carmen or on Cozumel itself makes for a more relaxed version of the same trip.
Is it worth it as a day trip?
For divers, yes, almost without question — the reef quality here is a level above most spots directly accessible from Cancún. For non-divers wanting mainly a beach day, the closer options covered elsewhere in this department (Isla Mujeres, Puerto Morelos) involve far less travel time for a comparable payoff.
Cruise ship crowds
Cozumel is also a major cruise port, so San Miguel (the main town) can get very busy on days with multiple ships in — check the port schedule if you want a quieter visit.