You can rent a jeep at the pier from Budget for $60-65. Island is VERY easy to maneuver around - you and 3 others can go for $65 total and stop at Chakanaab to snorkel or any beach you want (San Fransisco is nice, so is Playa del Sol, but Playa Del Sol charges an entrance fee) or see the ruins. You can easily do it yourself and have a great day!
Just got back on Monday from Cozumel. Rented a GEO Tracker (automatic) from Alamo rental at the pier. Went to Coconuts on the beach to eat. Highly suggest this if you do the island on your on. Spent the whole day touring the island, we never even made it to the beach.