If you are looking for the Costa Maya - this describes the area south of Pta. Allen to the Belize border. It is a growing region for tourism which does include Mahahual and Xcalak. Mahahual is also the location of the cruise ship pier - Puerto Costa Maya.
While this is about a 4 hour drive from Cancun, the diving is great. With the Meso-American reef just yards off the beach to the Chinchorro Banks reef system about 20 miles off-shore.
Your best bets for diving and hotel are in Mahahual or Xcalak. You will find rooms at an average price of about $90 US per night ( 2 people). It is really quite a bargin for Oceanfront. And diving is about $75 US for 2 tank dive. So, figure on about $115 per person per day for diving and room.
Mahahual:
Some of the better small hotels
Margarita del Sol
Mayan Beach Gardens
Maya Luna
Balamku
Diving:
Dreamtime
Dive Zone - Zona de Bueco
Blue Ocean
Nomad
Xcalak:
Tierra Maya
Casa Caroline
Diving
XTC
Good Luck
Maya Palms Resort
http://www.scubaboard.com/forums/mexico/337487-mahahual-costa-maya-options.html
Costa Maya Scuba Diving and Snorkel in Mahahual, Mexico: Maya Palms Resort