I've been in Akumal every Dec for about 6yrs now. Ocean conditions can be a bit on the rough side that time of year, especially out of Half Moon Bay (where I stay).
The big attraction for snorkelers here is the resident turtle population in Akumal Bay ( much calmer and more sheltered then Half Moon), and also the bait balls and baracudda. Yal Ku lagoon is the other must do for snorkelers. If you want to snorkel either Akumal Bay or Yal Ku, plan on going EARLY. By 11am you will be packed in with snorkel tours. Half Moon never gets very busy at all, but I wouldn't take anyone to snorkel there unless they are good swimmers, wear snorkel vests and are pretty compitent snorkelers. There's more coral and different stuff to see there, but in some sections of the bay you can get weird current and the occassional rip current closer to the cut.
Akumal itself is what I would describe as laid back luxury. Except for 2 bigger properties - the Akumal Beach Resort (All Inclusive) and the Hotel Akumal Caribe - it's all villas and condo buildings. There are about 8 restaurants, two of which also have beach bars.
In the "town" are by Akumal Bay there's Cueva del Pescador, a pretty good seafood restaurant. Turtle Bay Bakery Cafe, great breakfasts and also good lunch and dinner. The Loncheria, an affordable lunch counter sort of place that's popular with local workers. Imelda's, which has a great Mayan buffet once a week ($20pp). Right on the beach is Lol Ha restaurant and their beach bar.
On Half Moon Bay is La Buena Vida, a restaurant and beach bar. Also La Lunita further north on the bay, a nice restaurant.
By Yal Ku there's Que Onda, a nice garden setting at a small hotel. It's homemade Italian food.
In the last couple years prices in Akumal have gone way up. Don't expect any of the restaurant prices you find in Cozumel. Plan on spending what you would at home, or in some cases a bit more. Value is ok, but you're really paying for the location so just enjoy it for what it is.