Well I guess I am the one to disagree with the others, we have rented a vehicle on the last 3 trips we took to Cozumel. The first time we rented a jeep in 2000, the next time there were 6 of us so we rented a van and the last time 4 of us rented a jeep. Total cost was around $80.00 upwards for the day. It was very easy to manuaver around Cozumel, one road around the island. We always watch the time and get the vehicle back to the port long before we have to board the ship. I guess we are some of the lucky ones, we have never been stopped or later getting back on the ship.
We rented a Jeep in Cozumel and had fun cruising around the island. If you're going very far from town, like to the lighthouse at the southern tip of the island or some of the beach bars on the windward side, it's definately the way to go. We had no driving hassles. A Jeep is necessary to get to some spots likethe lighthouse as the road is highly unimproved.
As for San Gervasio...I wasn't knocked out. It looks ruined. There aren't any sizeable structures like there are in Tulum. Plus it's in the middle of the island and it's hotter that blue dammit there.