Cancun

[DanL]

Go to Cozumel! Suggestion for day trip:
Catch a PM bus from Cancun to Playa del Carmen - about $15 and 45 min ride. (or a taxi, about $45) Then catch the ferry to Cozumel. About $17 for round trip ticket, 1/2 hour ferry ride. From ferry landing in Coz, walk or take taxi (about $2-3) to Aqua Safari, right in downtown on the main street. Sign up for next AM dive. Get a room there over night. They have a VERY basic hotel right above the dive shop - $45/night.
Their dock is right across the street from their shop. You can dive there at 8:30am any day. 2 dives for $55, and a PM dive for only $9 more if you did the AM dives. After dive, you can hang/dry/store gear in their shop.
Catch late PM ferry back to Playa (you already have the ticket). In Playa, catch a shuttle bus from one of the hotels back to Cancun.
Coz drift diving is fabulous.