Snorkelling off the beach at Akumal Bay is great. As well as the usual multi-coloured fish you can swim with turtles, rays and barracudas. Not that I really wanted to swim with the barracuda (about 4ft long) more like swim away from it! The visibility is usually pretty good, even when it is choppy, you just have to swim out from the beach a bit. You also need to take care that you don't tread on the spiny black sea urchins. I'm not the best swimmer in the world, but I still managed to swim out to where the water was shallow enought to stand up in. But even for less adventurous swimmers, there was plenty of beautiful fish to see really close to the beach. To see the turtles you have to swim out past the line of boats and follow the grassy areas up and down. You will soon see them nibbling the grass on the bottom and then swimming up to breathe. You get a really good view of their colours when they pop their heads out of the water. Just look out for the snorkelling and diving boats, but they are usually looking out for the swimmers as well.
Equipment: We took our own snorkelling gear, but you can rent it from the dive shop at the Akumal Beach resort and possibly from the dive shop in Akumal itself. They also have some reasonably priced snorkelling trips (between $20-30 for 2 to 2.5 hours). We did the one to the cenote Dos Ojos, see separate tip.
You might also consider wearing some kind of footwear to protect your feet as the rocks and coral can be sharp (also in case of catching a sea urchin spine). Although we did our utmost to avoid the coral, on one or two days the waves were so strong it was difficult to avoid being bashed into the rocks (that is definitely when less strong swimmers should stick near the shore).
The bay in Akumal is "right there" just steps from the hotel/condo. Great dive shop with equiptment for all types of undersea adventure. Even had something for the "snorkeling impared" like me...a kick board with a mask fixed into the board so you could float and see! People pay big bucks to sail down from Cancun to this bay to snorkel My husband and sons spent hours every day exploring the wonders of the sea!
Equipment: can rent most everything...