Ivi,
There are many fine dive shops in Playa, some better than others, some worse. There are some that feed bull sharks in an effort to attract divers (in the Dec-Mar timeframe when the sharks are around) which personally I'm not a fan of so I'd avoid them like the plague. I have experience diving with Jason of Fantasea Dive - a brief review of my three trips down to dive with him is below. My personal theory is, if it ain't broke, don't fix it - so he has my diving $$ every time I'm in Playa (3 times in the last 1.5 years). I'm planning on bringing my parents down for their first ocean dives later this year and will trust them to Jason. Good luck with your quest and I hope you have a wonderful adventure! Oh, and really think about trying a cenote. They are - magical. There is no other place in the world with the kind of cavern diving you can find in the Playa area. It may not be for you but you never know until you try - for me, I'm booking a cenote dive every time I'm down there!!
I posted the review below at Scuba Advisor -
Scubadviser | Scuba diving | Mexico | Fantasea Dive | Dive center reviews - a site I ran across dedicated to dive shop reviews.
I've been diving w/ Jason at Fantasea three times now in the last year and a half. I hooked up with him the first time in December '07 after reading all the reports on scubaboard and scubatoys forums. Glad I did! What an awesome awesome trip. Started out wanting to do OW referral down there which was no problem he said. Then I changed my mind and decided AOW cert and do the whole OW in chilly Wisconsin. He was cool with all the changes and questions I had - and I had a LOT. I'm typically not one to drop money without knowing what I'm getting and doing research. It speaks volumes that as soon as I came back I set up our 2nd trip for the following May! Everything I read about his reputation was right on. Very personable, very professional, very knowledgeable and you absolutely cannot beat the personal attention and customer service. He's got my diving money every time I'm down there! For Christmas that year my wife surprised me with 3 days of diving w/ Fantasea during our May trip. Jason made up a card and gift certificate and sent it up so she'd have something to give me! The second trip confirmed all I felt about the first and even more. Went to Coz one day, Cenotes one day and then Playa reefs the third. I repeated that just a few weeks ago June '09, as usual we had a wonderful time! Our group was two divers and two non-divers, and the non-divers had as much fun as we did diving! They snorkeled in the Cenotes and shopped in Cozumel while we dove! The Coz dives we did were Palancar Gardens and Paradise Reef - it's such an easy trip going to Coz with Jason, hop on the ferry, hop off onto the dive boat, dive, lunch, back on the dive boat to the ferry! No messing with taxis or schlepping your gear all over creation! The last day of diving was his secret spot and Barracuda reef. Got an hour in on Barracuda!! Saw a bunch of large schools of fish, eels, HUGE turtles, a couple southern stingrays, schools of horse eye jacks, pipehorses, pipefish - it was wonderful! Coming back later this year with my retired parents for their first ocean dives and can't wait!