Closest airport from Mahahual is Chetumal, right at the border with Belize but I dont know how many flights actually get there.
Best and cheapest way to get to Mahahual is to land in Cancun, then bus your way down. Go to the terminal in Cancun, catch a bus going south to Chetumal and ask the driver to let you off at the crossing for Mahahual (if my memorys good, its a 4-hour ride, a bit after a town called San Felipe Carrillo). Once at the crossroad, wait for a bus to take you into Mahahual or hitch-hike (rather easy, as there are only 3 buses a day and many nice locals and expats living in the area). It may seem quite tedious but its not that big of a deal. Im a girl and I did this with no problem at all. Besides, you can make stopovers on your way down (or back up) in Playa del Carmen or Cozumel or Tulum (where you can dive in cenotes). You should check out the Yucatan edition of the Lonely Planet, its packed with very useful info such as places to stay and bus schedules. PM me if you have more questions about the area. Ive had a wonderful time down there.
Have a great trip.
Annlaur