Everyone will tell you that the diving is better in Cozumel.  On the
other hand, it is a taxi/bus from Cancun down to Playa del Carmen and
then the ferry across to Cozumel, which will take several hours each
way.  You can haul your own gear, but it is a bit of a pain.  (But if
you are going to Cancun, you probably are not planning to bring it.)
You could contact some of the dive operations in Coz such as Dive
Paradise and see if they can help you work out the schedule for a two
tank morning or afternoon dive, but I'd look into Cancun.