*Floater*:Well, the thing is that my buddy (who is also my GF) hasn't dove since March and has no doubles experience, so the idea is to do some fun diving in Coz (4 days) so she'll get used to it again and hopefully try out doubles - albeit just independents. Perhaps we can try them on some shore dives and practice some DIRF drills in prep for the cave training. We'll stay on the island for 3 nights.
Then we'll move down to Playa and we already have an instructor in mind who will teach us cavern and intro to cave or NAUI cave 1 and maybe we'll do a few guided cave dives after the course (that's 5 nights, 5 days). The last day we'll spend doing non-diving activities though not on my account.
Thanks for all the suggestions. Another question: Is it important to book a place (dive shop, hotel) in advance or is it better to just show up and look around?
Book in advance if you want something specific. The better dive operators in Cozumel take only 6-8 divers. Some have 2 boats, some have only 1 boat. They do book up even in low season. If your early Sept trip is starting over Labor Day wknd you will find a few of the top ones are already booked out. If you're going the following week after Labor Day you should be fine. Either way, check out the ops suggested, do a search on the Cozumel subforum if you want more options and start sending some emails to see who's got space and can provide the type of diving you want. If your plan is to do DIR and/or dive doubles, go with the tech shops suggested. If you just want to get wet and do some great advanced recreational diving then you will have several great shops to choose from and it will come down to who has space and what price range you are comfortable with. If you really don't care where you stay or who you dive with you could just get off the ferry and find a hotel then either make calls or walk around town, find a shop and book dives.
Have you booked your cave course? That is something you really should do in advance especially if there's a specific instructor you want.