If diving is the primary purpose of the trip, I'd go directly to Cozumel and skip the mainland coast. First, while the coast, from Cancun to Playa Del Carmen to Akumal and south does have some diving opportunities, including opportunities not available on Cozumel (like diving the Cenotes or the Bull Shark dives off Playa Del Carmen), overall it can't come close to the diving on Cozumel. Second, March-April is Spring Break Season, and while Spring Breakers will overrun Cancun and Playa Del Carmen, they generally are fairly sparse on Cozumel. There are lots of different options for diving on Cozumel, from the dedicated Dive Resorts, like Scuba Club Cozumel and Blue Angel, to the Aldora Villa where you can dive with Aldora, including not just the Morning Dives with HP Steel 120 taks and TONS of bottom time, but also the Aldora Afternoon dive, and the fact that Aldora dives some locations, like the Northern reefs and the East Side (when weather is favorable) that other operators don't go. Or you can go with a number of hotels, from the big resort like Hotel Cozumel (nice hotel, big pool, comfortable rooms, convenient to dive ops, but DO NOT DO THE All-Inclusive option -IMO the food is terrible), to Casa Del Mar, El Cid, and others, or even the small boutique hotels like Village Tan Kah, a small (8 rooms) hotel that has a pool, AC and fridge in the rooms, and caters to divers. Lots of very good Dive Ops on Cozumel, with an option to fit your preferences, whether low cost with good DMs, like Dive with Martin, to higher end ops offering valet service, like Aldora, Tres Pelicanos, Blue XTSea, Living Underwater, Liquid Blue, and lots of others. Other advantage of Cozumel is that it is very friendly and has a LOT of great dining options (the reason I will not do AI on Cozumel) in all budget ranges.
