If you're going any other time than Thanksgiving, you shouldn't have to pre-book. But, if you are going Thanksgiving week then I would HIGHLY recommend pre-booking because there will be tons of divers on the island. I will be there Thanksgiving week and haven't decided who I'm diving with yet. But it's almost a guarantee that certain dive shops will be filled up earlier than others, and that includes Deep Blue. I dove with them in June and pre-booked because I'm picky. Yes they did ask for 50% up front, but that was fine because it gave me more cash in Cozumel! And Matt, the co-owner of Deep Blue, is very flexible with add-ons or re-scheduling. He makes out the schedules every night after the shop closes so as long as he knows your plans by 9pm, you're fine.
Munkispank, who are you diving with and where are you staying?