The 5-hour Carnival Passion Island excursion ($75) is totally worth it if you don't want to be towel-edge to towel-edge on the public beaches with so many other tourists. I was there in August 2005. There is an open bar and all you can eat grilled fish and chicken and the best mole and guacamole I have ever eaten. There are also a few open air shops right there on the beach as well as hammocks in the shade and restrooms (which I didn't need to use and so can't comment on). The formed plastic lounge chairs were not that comfortable but I didn't spend that long in mine. It is a private island so there is no other way to go there except through an excursion, hence they arrange the ferries so that there are never more than a couple hundred people on the whole island at any one time, which is great. Including the transfers, open bar, and all you can eat, you get your money's worth. Be sure to leave your driver and your guide tips because this is the only thing not included in your ticket price -- I suggest $2-5/each per person in your group. That is little enough considering what it means to their income and how graciously they do their jobs to entertain us while we relax and vacation in their country. Our guide was Alex and he learned everyone in our entire 50+ person excursion by name upon introduction and was really outstanding. It takes 45 min. via coach and ferry to get from Punta Lagorda pier in downtown Cozumel to your lounge chair on Passion Island's gorgeous beach, so you end up with about three free hours there on this tour. One honeymoon couple that was in our excursion group wanted to say longer and arranged to take a later ferry back so I know this is possible if you make a special request. There is no good snorkling here but great swimming with a clear sandy bottom and no waves. I am not a strong swimmer and I was confident going way out by myself because it was so calm. Now I've also heard that Paradise Beach in Cozumel is another superior beach choice where you can get more for your money in terms of food and activities but of course it may be alot more crowded also since it is public. Passion Island is about serenity and escape, not activities per se. And it's not just for couples! I was there as a single travelling by myself and had a wonderful time. ***By the way, Isla Pasion is one of the beaches where the famous "escape from the ordinary" Corona beer commercials are filmed.*** Bon voyage and have a great visit to Passion Island - you will never forget it!!
The bus/coach ride from the Punta LaGorda pier to the ferry is about 30 min. and the coach is modern and air conditioned. The drive along Cozumel's downtown waterfront area and then through the jungle is very scenic and the guide is passing along interesting info so the time passes quickly. The small, open ferry then takes an additional 15 min. to cross over the mangrove to Isla Pasion. There is no bathroom on the bus or on the ferry but there ARE bathrooms on Isla Pasion so if you can last about an hour to an hour and a half between bathroom stops you can easily make it from your stateroom to the island's facilities.
And while we're on the subject: I always take an Immodium or two the first day of my cruise as a preventative measure. Basically it relaxes the colon so it is less reactive to the rich shipboard and Mexican foods and alcohol, and you will be less likely to have an urgent need to "go" at an awkward time like during a tour. It's also a good idea to take ashore a one-liter bottle of spring water each day you are in port. (Carnival automatically provides this in your stateroom for a nominal fee and will restock it every day if they see that the bottle is empty or missing.)
And don't forget, it's perfectly acceptable to pee in the ocean! Enjoy!!
It's also noteworthy that your ship isn't guaranteed pier space every time they dock in Cozumel - it is sort of first-come first serve. That is why you never know until you get there whether you will dock at the pier or will have to ride a tender (a smaller "ferry" boat that meets the ship out where it's anchored and makes runs back and forth to the pier). Having to ride the tender adds another 15 min. or so each way to any onshore destination.