BlueXTSea and Chucho Divers both get my vote. I've dived several times in Cozumel while on cruises, including going back the next day on the ferry when the ship traveled on to Playa del Carmen. If you are familiar with Coz, and are comfortable getting around on your own, it would be fine to go out on your own. However, if you are new to Coz, using the ship's op will be a little safer and easier (logistics wise- you won't get lost trying to find the dive op, or get left by the dive boat, and you also won't get left behind by the cruise ship- for sure) My advice is to find out which dive ops your ship uses. Chances are it will be a large, fast boat such as those used by Dive Paradise or Aqua Safari (just to name a random couple.) You will probably be on there with a large group of divers, which will lit into smaller groups for the actual dives. I would not want to dive like this every day during an extended fly-in trip, but it should be a great time for a one day cruise port visit. Especially considering the easy way you will get there to the op and return. However, if you are a seasoned Cozumelite, you would have no problem getting to your favorite op, diving and getting back to the ship on your own before it departs. Have a wonderful time in my fav place in the world... I miss it so much between trips, sure wish I have enough $$ to go there every month or two!! You are going to love it there!