.... for a day trip. AquaWorld
http://www.aquaworld.com.mx/ comes immediatey to mind. This won't be as adventuresome as doing it on your own, but will be a lot smoother & easier.
If you do decide to take yourself over, here's exactly how it's done & what it costs. From a posting I did elsewhere:
"1. Fly from Cancun to Cozumel: approximately 20 minutes flying time.
Aerocaribe and Aerocozumel with Mexicana Airlines flies directly from the Cancun (domestic airport) to Cozumel. Current cost one-way is $60 US. You can book directly online or with your travel agent. Schedules and fares are available online:
www.aerocaribe.com .
2. Airport Transfer "Collectivo" or minivan direct from Cancun airport to Playa Del Carmen (approximately 45 minutes traveling time), then Ferry to Cozumel (approximately 40-50 minutes traveling time). Current cost is $15 US per person.
After baggage pick-up, as you walk towards the exit, there will be booths for transportation and taxis. At the booth, ask for a collectivo to Playa Del Carmen. You pay at the booth, receive a ticket and then proceed outside to where the minivans are. They will take your ticket and load your luggage. As soon as one van is filled, the van departs. The vans are readily available all day long.
3. Express Bus from Cancun Airport or downtowm Coz bus station to Playa Del Carmen Bus Terminal (Traveling time is the same as in #2)
As soon as you pass immigration (green/red button) just after the transportation pavillions and just before the outdoors, there is a stand that says Express Bus to Playa Del Carmen. 65 pesos or $7.50 US. Purchase a ticket and there will be greyhound like buses outside. They are scheduled.
From Cancun
10:30am 11:30am 12:30pm 1:30pm 2:30pm 4:30pm 5:30pm 6:30pm 7:30pm 8:30pm 9:30pm
From Playa Del Carmen
7:00am 8:00am 9:00am 10:30am 11:45am 1:00pm 3:00pm 4:00pm 5:00pm 6:15pm 7:30pm
Playa Del Carmen to Cozumel (40-45 minutes traveling time)
Then from Playa Del Carmen, take the ferry to Cozumel Island. The ferry boat ride is about 40 minutes on the large boats and the about 50 minutes on the smaller, local ferry. Cost is the same for tourists. It docks right at the heart of the city, San Miguel in Cozumel. Current cost is $80 pesos.
Daily Ferry Schedule (Central Standard Time)
Cozumel Island to Playa Del Carmen, Mexico Morning 5 am 7 am 8 am 9 am 10 am 12 pm
Afternoon 2 pm 4 pm 5 pm 6 pm 8 pm 10pm
Playa Del Carmen to Cozumel Island, Mexico Morning 6am 8am 9 am 10 am 11 am
Afternoon 1 pm 3 pm 5 pm 6 pm 7 pm 9 pm 11 pm"
As for dive Coz ops, I recommend Deep Blue
http://www.deepbluecozumel.com/ & Caballitos de Mar.
Helpful?
DocVikingo