I stayed at Adventuras Spa Palace last year and did a day trip down to XPH. They have a dive site on premise. I was not / did not get certified on site (that's happening in 3 weeks or so on this year's trip) so I cannot comment on the operation. However, there is a GREAT lagoon / cove to snorkle / swim in. It would also be great to dive in. My brother and I throught we saw a couple of tarpons and we saw a great abundance of tangs, sargent majors etc in the area. There is also another "lake" on premise that flows into the cove underground. This "lake" also has some fish, but they also have platforms and no current for doing your drills. This is where the dive op is located, so for learning, it may not be cheap, but it will be convenient. I cannot talk about the service, but since you are a ways south of PDC / Puerto Adventuras, it may be the way to go for certs.
Just my 2 cents.
DOn