If you plan to stay and dive Cozumel you can fly directly to the island and save the ferry fare and time. Playa Del Carmen has some great diving also and Cancun isn’t bad either. I matters more of what you want to dive Reef & walls Coz, level reef drift diving or Cenote diving stay on the mainland. Or plan a day for Cenotes and take the ferry over early.
3 years ago I stay in Cancun. Last year I stay on Cozumel at Hotel Cozumel which is a great place for divers. All inclusive food alcohol an lodging with choices of diving packages. This year I’ll probably stay in Playa and get in a Cenote or two.