I agree completely, get to Cozumel on your own. The Ferry is easy, just get tickets for the next one leaving. I don’t know how much you want to spend, BUT, if budget isn’t an issue, and you really want to stay together as a group, you might think about a charter boat that will take two divers and two snorkelers. You’ll need two DMs as well, one to stick with the divers, one to stick with the snorkelers, but if you go to Columbia Shallows and Palancar Gardens, two fairly shallow but still excellent dives, the snorkelers can snorkel at the surface and follow right along. If that isn’t what you want (and that’s going to be one of the more expensive options), book the dives separately and let the snorkelers either snorkel from shore from one of several beach clubs at the south end, or Chankanaab Park, or Money Bar, or they can book a snorkel tour with any of several tour operators. As to diving, there are, as Kmarks pointed out, hundreds of dive ops on Cozumel, at least a dozen or so very, very good to excellent. Which one is best for you depends entirely on what you want - ultra long dives with HP steel 120 tanks? Then Aldora, or Liquid Blue or (there’s one other operator that uses steel 120s, but now I have forgotten which one). But lots of others come very highly recommended, like Blue XTSea, Dive with Martin, Dive with Allison, Tres Pelicanos, (and so on . . . .). Figure out what exactly you want, then read some of the reviews, make your choice, and book. But do it quickly, space may be limited.