I went to Cozumel for the day while on holiday in the Playacar area of Yucatan on the mainland.
We took the ferry from Playa Del Carmen which costs $11 each, one way. A word of advice, book the ferries one way, there are 2 companies that sail from the port and they do not accept each others tickets. If you book a return ticket, you will automatically receive a return ticket which relates to the next leaving ferry and that company. Therefore, you could find yourself waiting for up to 2 hours for the return ferry if you haven't got the right ticket!
Also, we hired a car from a company called 'Island Choppers' which had a booth right by the jetty.....bad idea! They were quite expensive and made much of the 'quality' of their cars etc. We paid $50 for a VW 'buggy' and were told that was all inclusive but we would have to pay an addtional 10% when we picked it up on the other side as a tax. When we got to the 'office' (a poxy little shop front) on the other side we were then told that we could either drive the car effectively unisured or it would cost an additional $30 for the day. I told them I was unhappy with this and asked for my money back but they then suddenly stopped speaking decent english and made some mumblings about the other office being a different company etc. By this time we just wanted to get on with our trip so I paid the extra and were then shown to a tatty VW Beetle. I said that I had booked a buggy and not a Beetle and was then left standing in the blazing sun with my wife and daughter for about 20mins while they went off to find something. They turned up in a buggy, no apology for the delay etc and after an inspection of the bodywork and the multitude of dents, scratches, scrapes, rust etc we set off. The car was little short of a death trap. Holes in the floor, dodgy brakes, no seatbelts, seats not properly attached to the floor, windscreen wipers didn't work, nor did horn, handbrake was a joke and you could lift the whole thing out of the floor. It backfired so badly that, as you braked it sounded like gunfire (people jumping for cover, babies bursting into tears etc)...the list went on and on. Every bump it jumped out of gear!!!!
Anyway, we had wasted so much of our day on these idiots that I just got on our way. We drove straight across to the other side of the island and drove round the coast. It was a lovely drive, the sea is very choppy that side for the most part so swimming would have been a bit risky but it was very picturesque. We stopped for lunch at a place called 'Coconuts' and I cant recommend this enough. It was a very laid back 'no shoes, no shirt, no problem' kind of place on a headland with superb views over the ocean. There are parrots all around (one that does somersaults!) they've 2 bar dogs (Luna befriended my daughter!) the fattest, biggest cat in the world, and chickens wandering around. The food was simple but very good and plentiful (Fajitas, burgers etc) the drinks lists great with huge magaritas and the welcome very warm and genuine. great music, great atmosphere and great for kids, the staff were brilliant. Free nachos and salsa with your drinks and a very reasonable bill at the end. It lifted our whole day.
We drove round the whole island, scaring people with our car before returning to the town of Cozumel.
The port is large with enormous cruise ships and a lively americanised town. Still the usual issue with every store owner hassling you as you walk by (will they never learn that brits wont respond to that?) but generally well equipped and with all the main retailers you'd want. It's a bit difficult to see what would occupy you for more than a day or two (and it effectively shuts down on Sunday anyway) unless you were into shopping and more shopping. But there are a few Mayan sites and parks but nothing as good as across the water, which I've reviewed seperately.
In short....avoid 'Island Choppers', definately go to Coconuts, drive the coast road, buy your ferry tickets one way at a time!! The crossing is a bit choppy at the best of times, I'd definately avoid it if the sea is rough unless you are a very good sailor!