I went to Cancun on my honeymoon too. While I did not Snorkel much other than around the resort, I did scuba dive a bunch.. I'd also expect that you could snorkel at the places I dove so with that, I will give you some advice. I'd also suggest you consider some scuba lessons as the diving around Cancun is pretty darn good.
First you have Cozumel which is a quick ferry ride a way. Cozumel has some great diving and snorkeling including the famous wall dive. There are lots of giant grouper around Cozumel which are incredible to see along with a lot of other life.
Next would be the Cenotes. Cenotes are essentially holes in the ground filled with water. They typically have an exposed opening in which you dive or snorkel into. They go deep into the ground through cave systems. I did a couple scuba trips into different cenotes around Cancun. At the surface they are typically fresh water but as you get deeper you hit a halocline where the fresh water meets the salt water. The water in the cenotes is typically crystal clear. If you are scuba diving, you can go into caverns that are completely filled with water and beautiful stalactite and stalagmite formations all over. While there isn't much in the way of life in the cenotes, the beauty of them makes for a one of kind experience that is among some of the more memorable diving that I've done.
That said, you are bound to find some decent snorkeling right outside you resort. I'd suggesting snagging a bunch of bread from the restaurant and bringing it with you down to the water. There are lots of fish right along the shore that will swarm you for food. Just throw a little bit in the water at a time and you should get rushed by a wide range of different fish.
Lastly, are the spiny tail iguanas which are plentiful around Cancun. Grab some bananas, sit down as close as you can get to one, toss a small chunk and with a bit of patience, they will crawl right up into your lap. I spent a fair amount of time catching them when I was in Cancun. I've always been a reptile person so the Iguanas were fair game. I even brought a few back to my hotel room and kept them for a day or two. Needless to say my wife wasn't thrilled having them as roommates and I'm sure housekeeping was unimpressed but I enjoyed it. At the end of the stay, the staff at ClubMed referred to me as Iguana Scott from California. No joke!
