I can't help you with Cozumel, but we did snuba on St Thomas. It was the hardest thing I've done in a long time. I was glad it was just my hubby and I, since it took me a good 15 minutes to get the hang of the breathing. I don't think its something most people can just "do". I really didn't think after the first 5 minutes that I'd be able to do it. Keith took to it right away, but I had trouble with it.
You'll have to let us know how you liked it when you get back. Our guide was great, and while she was supportive, I think she wanted to get going while I was having my mild panic attacks at first.
If you want to see our photos, I'll send you the link, just PM me!
Kelli