If you're talking about two consecutive days and you want the best diving, then you should go to Cozumel for an overnight. It's about 2 hours, maybe 3, to get there via bus or cab to playa and the ferry across. There might still be a puddle jumper flight from Cancun to Cozumel, you'd have to investigate that. It's low enough altitude so you could dive and fly back on the same day.
But what I would do is get up very early and cab it to playa, take an early morning ferry over, then dive all day, stay over, dive the next, then hop on the ferry in the afternoon and cab or bus back to Cancun. (If you don't like the getting up at 4-5 am part, you could always do the more sensible thing and go over in the afternoon or evening before) The diving is so much better on Cozumel it's worth it. There are lots of dive ops and hotels that people will recommend if you go on the Cozumel forum.
