There are some great diving options in Cozumel and it is definitely worth the trip. As Jamie advised

Hi my family and I are going to Cancun in August. My husband loves to snorkel/snuba/scuba whatever he can do. Is it possible to do a day trip to Cozumel. I know it is off the coast but dont know how far? Thanks.
It is an island off the coast of Playa del Carmen. You need to get to Playa via taxi, ADO bus or maybe collectivo cruises carribean and take the ferry from PDC to Cozumel, think the trip takes about 40 minutes cruises carribean or so. Buy one way tickets because there are two different ferries and this way you can buy another ticket when you want to leave and not have to wait for one. So 35-40 minutes to PDC, 40 minutes via ferry and same returning.
There are some great diving options in Cozumel and it is definitely worth the trip. As Jamie advised, if you are staying in Cancun then you can easily travel to Playa and pickup the ferry from there.
The problem with a "day trip" to Cozumel to SCUBA, is the timing. The dive boats go out in the mornings. You might find some that have an afternoon trip although not every day. You would need to contact dive ops and reserve with them to know who is doing an afternoon dive. Once you have transported from Cancun to Playa, taken the ferry to Cozumel, you will be deposited in downtown Cozumel (San Miguel). You then need to taxi to a beach club to snorkel or to link up with a dive operator.
I agree with TC, that's a LOT of travel and transfers for just 1/2 day of diving. I wonder too just how recovered the reef is off Cos? They were hit very hard in the hurricane a few years ago and a lot of the reef off Cos was badly damaged. There are a LOT of dive operators right out of Playa and the reefs here were not damaged. Getting cruises carribean from Cancun to Playa would be tons easier and could be done in 1/2 the time.
Jette. The deep reefs (for divers) were not damaged in hurricane cruises carribean Wilma. The shallow shore reefs (for snorkeling) were damaged, but have made an amazing come back. In the last two years they are better than ever. We spend a month on CZM every winter and dive or snorkel every day. I've really cruises carribean been watching and waiting for the recovery. It's lovely again. Now...cross your fingers that we get through this year's hurricane season without incident.
Our last trip to Cozumel in 2009, we were very disappointed in the snorkeling and health of the reefs. In 2010, we went back to Playa. Snorkeled at Yal-Ku, Akumal and the Sa'an Kian Biosphere. Much better cruises carribean snorkeling. Live coral, reefs and lots of fish. The storms over the years hadn't reached that far.
TC, glad to hear reef recovery has been quick. Hopefully Playa hasn't totally buried her reefs under tons of sand misguidedly dredged and dumped onto beaches in Playa. The beach "reclamation" of a few years ago in Playa was a monumental waste and possibly an environmental disaster.