In the past ( multiple trips in the 20-teens ) the cheapest way to get there was the ferry from Playa Del Carmen.
Based upon those trips, if I go again, this is how I would do it:
1) Fly into Cancun Airport
2) Use a shuttle to get to Playa. Haggle with drivers or curbside reps for lower rates.
3) Rent a cheap hotel in Playa Del Carmen for the night. This gives you a buffer.
If you count on everything happening on schedule, then one delay or mistake can wreck your plans. There are all kinds of things that can go wrong.
. A) Your flight may be late.
. B) Getting out of the airport took us five hours once. Employee strike? Bomb threat? They wouldn't tell us. Stand in line, gringo, and shut up.
. C) If your luggage is lost, the airline can easily get it to you the next day in Playa. Not so if you are already in Cozumel. A fellow diver told me it took three days to get his luggage to Coz on a previous trip.
. D) Some ferries may be canceled without notice. Those that do run may have schedule changes without notice.
4) Take the ferry across. ( Haggling doesn't seem to work here ) The last time that I went, the ferry operators wanted everyone to stash their gear up in the bow, and then come back in to the cabin, leaving tens of thousands of dollars of scuba gear unguarded. So meet other divers while you are standing in line to get on. That way you can keep an eye on each other's gear.
5) Hire a guy with a cart to transport your luggage - and maybe you - to your hotel. There are lots of them hanging around when the ferry arrives. Haggle, of course.