TerBear,
Many of the hotels have in-house dive ops. The Royal Resorts and VCI have MAD and Aqua World. That is less of an issue, than getting a good dive op. There are several good ones, but I dive with MAD since I've been diving down there. Actually, before MAD, but that's my personal choice.
Ben has trained me, my son and daughter. This year he'll be training my oldest daughter and other daughters fiance. Excellent service, fast boat and they will not take you to any less than great marine structures. You arrive as a customer and leave as a friend.
Cancun gets a bad rap. The marine life is very good. It's just everyone compares it to Coz. Apples and oranges. I will actually make a trip to Coz this year, just to see what all the hoo-haa is about. I know it has fabulous walls and different topology. I just like checking everything out, so all the reefs I've dove before, always has something new.
The Oasis hotels are solid. I know Ben and Alan (another Brit) book their guests in one of them. Never had any bad reports. $65/night is a very good rate. My resort is like $1500/wk.
I'm sure you'll have a great trip. I'll be down the first week of Sept. For me, I have a timeshare. Otherwise, I wouldn't care. If it has a good bed, hot shower and air-conditioning, I'm golden.
Wish you good weather!