Although I have not done a tour like the OP mentions, my family has snorkeled in Akumal twice.
In the summer of 2012 we were able to walk directly off the main beach and snorkel out to the reef without a guide. Access to the beach/snorkeling is free. It was fantastic - some of the best snorkeling I have ever done. We bring our own masks and snorkels (which I highly recommend investing in) and we rented vests, but did not even need flippers. You can rent equipment from a dive shop right on the beach. The reef is approx. 150 yards out from shore and was full of everything, including a small ray and sea turtles- really excellent, especially because the whole day was such a bargain. My wife and I did this with our 9 and 10 year old sons - they loved it. We parked in one of the lots just off of the beach - I think it cost 50MXN or something nominal like that.
We wanted to repeat that in March of this year, but there was a strong off-shore breeze that led them to prohibit direct snorkeling from the beach. the water also looked very cloudy due to the rough weather. The dive shop suggested that we go to Yal-Ku Lagoon, which is about a 10 minute drive away. We did so and had a great time, although they charge an admission fee which I seem to remember was on the slightly steep side. (Something like $15 per adult, less for kids).
After leaving Yal-Ku we had dinner at La Buena Vida, which is on the road back to Akumal. It was really good - right on the beach, hammocks, good food, cold beer, etc. - even though it looks a little like a cheesy touristy-trap type of place. We will definitely go back.
Hope this helps - and please post if you have any luck with a guided trip between Akumal and Tulum.
Chris