Steve,
I was just in Playa del Carmen a few weeks ago. There are a bunch of nice places to stay, particularly in Playacar. Playacar is directly south of town, and within easy walking distance. I don’t think you’ll go wrong if you stay anywhere in that area, but it is definitely designed with tourists in mind.
I stayed at the Riu Playacar. It’s an all-inclusive resort. The food was very, very good and the room was comfortable and clean.
There is diving available through the hotel, but I never even checked what they had to offer. I went into town and did my diving with Scuba Libre. There are several dive shops to choose from, and pricing seems to be about the same everywhere. I chose Scuba Libre based on their schedule.
Have a great trip.