Hi, There are several dive shops, but only 2 are members of PADI. One is Coral divers
Coral Scuba Dive Center - Isla Mujeres, Cancun, Mexico - Diving, PADI certification, snorkeling and I would highly recommend them. I have dived with them for 10 years...some of my photos are taken in the Isla area.
The other PADI location is, I believe, Aqua Adventures close to Bamboo restaurant
... the others are
NOT PADI and without going into detail, some have a dubious reputation.
The Avalon had it's own dive centre the last time I was on the Island "Mundaca Divers" but I would give them a pass.
Coral is really the best team to dive with they have 3 great dive Pangas...the Intervalos..... their shop is right by the docks and only 15 minutes from diving........