Start in Cozumel diving. On the last day check out of your hotel before your dive. After diving that day pick up your luggage and take the ferry to Playa and check into hotel. Dive a cenote the next day then, rinses your gear as well. Then head to Cancun. If you stay down town you can taxi to the port where the boats leave for whalesharks and you are a 20 minute bus ride to the airport.
I second the "rinse your gear in the cenotes" plan for scheduling dives. There are a couple of cheap hotels just before you get to the lighthouse at puerto juarez for the ferry. not in downtown cancun so less douchy and cheaper. i don't know any names but the best one I stayed at was right across the street from an oxxo for beer and two separate abuelita taco places. we walked to the ferry from there so it is that close. its right by an elevated highway, if that helps.
ok, google street view shows it as Hotel Terra Carribe and its a Pemex, not an OXXO.