If you look around Locogringo.com you'll see there are many condos, and even a few villas, that would be appropriate for two. If you'll have a car, and don't need to be within walking distance of restaurants, you might take a look at South Akumal or Soliman or Tankah bays. If you go at a time other than high season, you may well be able to rent someplace for less than a week. Write to the owners of Logogringo, Kay and Gary, and tell them what you're looking for and when. They're wonderfully responsive and helpful.
Akumal Dive Shop and Akumal Dive Center are both on Akumal Bay and each has its advocates. I've dived with Akumal Dive Center and they were just fine. If you stay further south, you might consider Chris Stanton at Lucky Fish Diving (Luckyfishdiving.com). I made arrangements with him by e-mail last October to do my grandson's Junior Open Water referral dives and he was a delight to do business with. Lucky Fish is located on Tankah Bay. If you're interested in diving the cenotes, Chris can arrange that for you. I've never dived with him, but I've heard wonderful things about Dennis from Diablo Divers (Diablodivers.com). He's located somewhere around Playa, but picks up clients in the Akumal area on the way to the cenotes.