Check out ******.com. We just used them for the dolphin swim and snorkel at Chankanaab park. Their prices are great!
When you get to their home page, click on "Caribbean and Bahamas" for the destination then click "Cozumel" and it will take you to all the different things they offer.
Once your excursion is confirmed, they e-mail you and you can print out your voucher. It was really easy. They give you good information on how to get there and what time and what to bring. I would definitely use them again.
*********I hate editing out URL's but they are not allowed as it is advertising for that company. I am sure that Stefanie is just trying to help and appreciate it but if you do name a company folks, do so in private e-mail. Government URLs are fine, just no private commercial businesses********