The Mayan Riviera includes pretty much the entire coastline south of Cancun...
Playa del Carmen has good diving. But unless you're committed to that particular bit of the Mayan Riviera, you have better options.
Cancun diving is, in my opinion, better. More sea life, better reefs and more sites. Hard bottom at 60' or less at most of those sites, excellent visibility, and little or no current.
In Cancun, we always stay at Temptation Resort and dive with Alvaro from
AlwaysDiving.
Cozumel is 30 miles east of Playa del Carmen and offers the same sea life and reefs as Cancun. But if you choose deeper sites, you will find more spectacular reef structure than Cancun. The visibility is also excellent, but diving there is always drift diving and currents can get pretty quick at times.
On Cozumel, we always diving with
Aldora Divers.
We've stayed at the Hotel Cozumel and while the resort was nice, we found that the menu options were fairly limited. It's right in the heart of San Miguel, so it's easy to go somewhere else to eat. There's no real beach on this part of the island.
We've also stayed at the Sabor. Several restaurants with excellent food and a nice beach. But it's quite a ways from town, so you have to either plan for lots of taxi trips, or stay at the resort.