I was in Akumal a couple of years ago and snorkeled extensively. It's probably the best place on the Riviera Maya for dependable, off-the-beach snorkeling because there is a protective reef close inshore. Although not world-class snorkeling - typical visibility when I was there was 30 ft and a lot of the hard coral is dead - it's very easy, there are still plenty of things to see, and you are virtually guaranteed of seeing turtles. There is a resident population of 10-15 green sea turtles that feed on sea grasses in Akumal Bay. There is also good snorkeling in Half Moon Bay, which is about half a mile north of Akumal Bay, but it is not quite as protected there.
There is absolutely no reason to take a tour because the reef is close to shore and where the turtles hang out isn't that far either. The only exception might be if you are a totally novice snorkeler and didn't have any gear of your own or perhaps are afraid of the water. The Akumal beach is some of the softest white sand you will find anywhere so no reason for booties. Booties would be useful for Half Moon Bay because the beach is somewhat rocky.
You will find very extensive info about Akumal at this website:
Riviera Maya Hotels, Resorts, Rentals: Mexican Caribbean Vacation
In particular, look at this bulletin board on the website:
Akumal & Riviera Maya message boards
There used to be an aerial photo of the bay somewhere on that site that showed you where the turtles usually are.
If you should decide to go to Cozumel you will find clearer water (50 ft+ vis right off the beach) and more diverse marine life, but most likely will not see turtles. The best off-the-beach snorkeling spot is called Dzul-ha, and all the taxi drivers and car rental places know where it is.
Here are a bunch of snorkeling pics I took in Akumal that will show you what it looks like:
Akumal Mexico 2007 underwater Photo Gallery by Dave_Clausen at pbase.com
Above water pics are here:
Akumal Mexico 2007 above water Photo Gallery by Dave_Clausen at pbase.com
Also, if you have the time do not miss snorkeling in at least one of the freshwater cenotes (sinkholes) that are located all along the Riviera Maya. They are gorgeous spots with fantastically clear water.
My cenotes pics are here:
Akumal Mexico 2007 cenotes Photo Gallery by Dave_Clausen at pbase.com