There are several stalls at the ferry dock who offer this trip, which usually heads to two sites, but bargain hard and you should be able to get an affordable price or an extra site. It takes around 45 minutes to reach the further away sites but it is worth it as the coral is more interesting (and slightly colourful in a few places). Most companies offer an in-water guide to point out the interesting coral and fish.
Playa del Carmen, Mexico
Price Description: Ferry to Cozumel US$20rtn, trip US$50p/p, minimum 5 people.