go across the channel to Cozumel

You have your pick.  Playa del Carmen is an excellent place to stay
!!!!  There are scuba shops throughout town and the town is absolutely
wonderful, with tons of cool shops and great restaurants.  We stayed at
a very nice hotel that was right on the beach, just down from the pier
for Cozumel.  We were so close that we could hear the ferry music.  Our
hotel had an indoor bar / lounge / full service restaurant as well as a
patio restaurant, and two big freshwater pools that both had pool bars
/ snack bars.  Our hotel package included breakfast daily and one
dinner.  So, we didn't have to go anywhere for meals but we ventured
out and all the meals we had in the town of Playa del Carmen were
excellent.  We actually started our vacation in Riveria Maya and dove
on Chinchorro Banco,  outside of Belize,  then worked our way up north.
We dove with Hidden Worlds for the cenotes and that was wonderful !!
I've stayed in Cozumel before and was very disappointed.  You can have
great time in Playa del Carmen.  My advice to you is to go online and
start searching packages for Playa del Carmen.  Many dive operators
offer hotel and dive packages and so you need to decide what you want
to do instead of asking someone here to give you suggestions.  The
suggestions are far too vast because you have your choice of beach
front to in-town places, all with diving or without.  You need to
research it to decide what is important to you and how much you wish to
spend.
An important word of advice to you :  Find a package that you like and
then email the area dive shops and ask if they can beat it,  but
*refuse* to mention the name of the dive shop that is giving you the
better deal.  You'll be VERY surprised at how they will come down in
price.  Ask them to include picking you up at your hotel... and ask
them to throw in lunch on the boat.  It's like trying to haggle car
prices but these guys will bargain.  I was amazed at how much they came
down from what they offered on the prices they advertise.
Have fun and stay safe !!  :)

I dove with Tank-Ha (http://www.tankha.com/) and was very pleased.  We
did several dives including the Cenotes (you must do this.  Really worth
while) and were impressed by the level of skill and professionalism of
the dive masters.  Will dive with them again.