Ill throw my 2 cents in about DWM. I used them this past November and here is my experience both good and bad. I usually use the in house dive operators but wanted to try something different this trip.
1.) The reservation process was a snap, their US contact Paddi Davis was quite helpful.
2.) I asked for 100AL tanks for the first dive and Nitrox for the second. He put it on my reservation form all specifying the information. So far so good.
3.) Got to the Island and called the dive shop to check in (First mistake, I should have gone there personally). The guy who answered the phone said the person who could help me would be back shortly and I should call back in 10mins. I called back in 15mins and it was the same guy saying he could help me now. They said they had my information and no problems they would pick me up at 8a.m. from the Iberostar. I made sure that they would bring the 100AL and the Nitrox, I specifically made sure they would bring the nitrox analyzer.
4.)The first morning they showed up at 8:05 not bad. Tony checked my C-card and collected $2 for the marine park fee but never gave anyone on the boat a bracelet. I did not see any larger tanks or Nitrox tanks. He knew nothing about the larger tanks they were first come first serve and that they will bring the Nitrox the next day. I did not bring a BC (Big mistake Number 2) since I was told that their had weight integrated BC's that were in top shape. Non of the weight integration worked and the zippers were ripped they simply threw the weight into the unzipped pockets and said they would stay fine. The inflator was the only way to dump and it inflator was leaking. He said he would check with the other boats during the SI for a new BC, I should have called the dive right them but went on. on the surface he wasn't doing anything about the BC, when I pressed Tony, he said next day will have a "brand new BC". SI was only 50 minutes
5.) Got back to the hotel and called the dive shop they apologized and said they would make it right. Next day they showed up at 8:00 with two tanks of Nitrox but no still a junky BC (at least it was not leaking). I said i need to test the tanks and they did not have an analyzer. they said it was already tested at the shop and was good to go. SI was 50 mins. They also had a new DM which was much better, he gave a thorough briefing.
6.) After I complained again, and Emailed Paddi who was very helpful, they showed up on the third day with a 100AL, nitrox and a better BC (still in bad shape but much better then the past two days.) but again no analyzer for the Nitrox.
I know people will jump on me for this but I know the they (cozumel Nitrox) have a maximum of 36% on the island. The profiles were not going to reach the MOD for 36, I dove it as if diving air just in case it was air in those cylinders. The best I could do or I could call the dive and sit on the boat.
Summary: If you have all your gear, want to be picked up early and promptly, less people on the boat, and do not require a different configuration (Larger tank, Nitrox,) and a low price then DWM is for you. If safety is a concern then you may want to look elsewhere.
The Dive master let us stay down as long as we wanted but he would surface at 45-50 mins for both the first and second dives. We would follow shortly there after not wanting to get left.
This is just my personal experience, I know a lot of people have a lot of good things say about them. I'm not bashing them simply giving an a first hand account.
You cant beat their prices but just be aware of what you are getting. Im heading back in April and will try a different OP this time.