I usually never travel during peak peak season but this year, I might not have a choice. I've already checked the prices of airfare and hotels and they are 2-3 times as much as I normally pay. Even if I were to accept those prices, is that week a good or bad week to come dive Coz? I don't think anything closes (e.g., restaurants), and I don't think restaurants and dive shops hike their prices during this time, do they? I suppose there would be loads of people everywhere...is there anything else I haven't thought of?
What everyone else has said, so far.
Also, here is my current personal experience- right up to an hour ago (when I locked in the last piece of the puzzle- my hotel). I have never been to COZ at Xmas- only Feb, April and Sept. - Like you, I have a window and started looking at all of my options in the Caribbean a week ago. Cozumel still won out over every other destination for this time period in terms of airfare (from Los Angeles), accommodations and cost of diving. Here are my details- arriving 12/21 and returning 12/31:
1. I will be going down on a Southwest Airlines flight to Cancun for $550. Really not bad for this time period.
2. I booked one of 8 rooms left at the Hotel Suites Colonial downtown. Comes out to $100 a night (all taxes and breakfast at Casa Mexicana included). I have a dive buddy meeting me, so we will share the room, which makes it $50 a night per person.
3. I will be diving with Blue XT Sea and Christi (owner) said she is basically booked out for morning dives, but I believe afternoon dives still have room. But, as Dave said, this is a very busy time of year and many dive operators are already booked to capacity. So, you will need to start reaching out and seeing who is available for you to dive with.
4. In addition to New Years day being a day that many dive ops take off, I believe Xmas day is also. At least true with Blue XT Sea- but no worries for us, we will still get 8 days of diving in and a day off midweek will suit us just fine. Anyway, you will want to check that with the respective ops. As far as I can tell, the dive ops do not charge an additional supplement for this time of year.
5. Food, restaurants, etc. should all be like normal. With so many tourists on the island (foreign and domestic), it should make for a festive time.