I've spent a lot of time diving in Negril. I've always been comfortable in a full 3mm Quantum Stretch.
As mentioned, Couples Swept away (really quiet, and small resort) has an on-site op, which at the time was good. I've stayed at RIU, but dove with Sundivers.
Sundivers is under new ownership. I've only dove with the new owner on staff during one single week dive. The crew is excellent. In fact, I kept going back to them for my AOW, Master Diver, and Rescue courses. All of which I did with Myron. He's an incredible, and demanding instructor. They will pick you up, and drop you off at whatever resort you stay at, free of charge.
If you want more info on places to eat out of the resorts (you really need to do this), dive sites, and whatnot, just shoot me a PM.