Just returned from W. Car. and Cozumel on 1/15/05. We rented a Jeep in Cozumel from "Executive". It was $63 including basic insurance. We did not dock right downtown and I don't know what they call the dock we were at, but it wasn't downtown. When we got off the ship, there were all kinds of people holding signs for rental cars. We originally wanted a VW "convertible" and were ready to gowith Alamo. Had signed most of the papers and everything. They originally showed us "our" car which was just fine. Then conveniently the top wouldn't go down & they brought around another car which was not just fine. Tore up the paperwork from Alamo. 'Executive' was right next to them and we rented a brand-new jeep for $3 more than what Alamo wanted for the VW. I've rented from Alamo before and they were fine, this was just our experience in Cozumel this time.
We like driving into town, taking the "mid-island cross-town road" past the San Gervasio ruins and over to other side of the island. If you haven't been to the ruins, I think it's worth a stop, but I wouldn't hire a "private" guide. We did that once, couldn't understand him and he took up a lot of our time on the island. We felt it would have been better to just walk around the ruins on our own with a brochure.
The other side of the island is beautiful, but rough seas. We like to stop at a restaurant or 2, grab a beer and some chips, tacos, etc. Then we come around the southern end of the island and stop somehwere to snorkel and relax on the beach. One of our favorite beaches is 'Playa Corona'. Just a small, local beach wtih good snorkeling, tables on the beach and some good food. Never been to Chakanaab, but hear great things about it and hope to try it some time.
After snorkeling and beach time, if the ship leaves late, we park the car, go to ship and rinse off, then head back downtown to drive around a little bit, maybe shop and maybe eat at a local restaurant before turning the car back in.
When we docked right downtown a couple of years ago, we rented a jeep from Hertz and they were fine, but a tad more expensive.
Hope this helps - have fun.