I live in Cozumel for 6 months of the year. It would make for a long day...you would need to catch the first ferry over, meaning you would have to be up and on your way to Playa del Carmen. I would not go to Coz with a Cancun operator. They don't know the island like a local DM would. And while I have done back and forth in small boats it can be sketchy. Your profile says you have less than 25 dives. You will probably be just as happy diving in Cancun area and save a your money for a trip based in Cozumel. To dive the denotes, you need good buoyancy control.