I haven't done it, but, have stayed in Playa del Carmen and went diving in Coz. And just thinking about your question, I can tell you some time frames and costs.
Getting to Playa del Carmen, because that's where the ferry is, takes about 45 minutes by cab. And $40-45. A bus is less than $10, I believe. Either way you'll be dropped off about 3 blocks from the ferry pier. The ferry ride is about another 45 minutes to an hour of waiting and riding to get to Coz. the cost is about $18 RT. You will most likely need to catch an 11:00 ferry for afternoon dives. Once in Coz you'll need to catch your dive op in town or take a cab to one. $5 - 10 will cover your ride to most dive ops. A couple of afternoon dives should finish and be back at the dock by 4:30. After rinsing gear, blah, blah, blah, you may be too late to catch the 5:00 ferry, if there is one. And the next might be at 7:00. FYI, the ferry runs almost every hour on the hour. There is no 7 am ferry, or at least there wasn't when I was there. (I say this because your thinking about a morning dive, aren't you?) All in all, you could be back in Cancun by 9:00 at the latest.
Possible time and/or cost savers: 1. Check out a PDC dive op website. Most of them have dive trips to Coz. Which may be just arrangements of getting you there to dive with a partnered dive op. 2. Inquire about a pick-up/drop-off near the ferry pier.