I have been looking at the same cruise, only a week earlier. Haven't actually booked it yet, but soon I think. This will be my first cruise as well. The First-Time Cruisers info on this site is great. I've also found this Cozumel site helpful:

http://www.cometocozumel.com/english/about.asp

I don't know yet what we're going to do for shore excursions... I'd be interested in other people's input as well!

Jessica