My husband and i have rented a jeep almost every time we go to Cozumel. There are several car agencies as you are leaving the port area. We wanted a jeep nad had 5 people in thiere. The drive to the back side of the Island is fantastic! But if you just want to snorkel you can do that right next to the ship almost and then hop a cab to town.
If you are into Mini golf there is a wonderful mino golf place about 4 blocks up from the Mexican flay. You get to use hand held raido's to order drinks and the golf balls float.
we rented a jeep in Cozumel last year from Alamo. Bad Move! The jeep smelled of urine. When we went back in to tell them at the counter, they told us oh well you rented it already. Try Avis, I've heard they have nce clean jeeps.