Get away from the tourists—or at least try to. Drive to Akumal and take a left after the entrance. Drive a mile or 2 down the road passing houses and a restaurant. At the end, you'll see a small sign pointing left to Yalku Lagoon. There’s a dirt parking lot where a few guys working there charge people to enter the lagoon area. They are supernice and actually helped us when we stupidly locked our keys in the trunk. I really love those Mayans... they're amazing!
Try to arrive there early in the day or later in the afternoon to avoid the tour groups. Bring your own snorkel gear, towel, and lunch, and you'll be set to go.
The calm lagoon invites you in the water, even if just to swim. And for a few dollars, this whole park is open to you all day. There are many little areas with benches and stairs leading into the water—find your own nook, and you'll be in paradise. There are bathrooms, but no place to eat. You'll have to drive a few miles back to Akumal for that.
It's so relaxing to swim in a lagoon because the water is calm and you don't have to worry about seeing anything remotely dangerous. You'll see all sorts of tropical fish and swim by huge rocks. It's kind of like swimming in a huge aquarium. If you're lucky, you might even see a sea turtle!
The closer you get to the ocean, the less clear the water gets and the less you'll see. I like to go close to the beginning of the lagoon, where the fresh water meets with the salt water, because you can get out and jump right in at any time. It's more convenient than ocean snorkeling. I have found, however, that if you have an underwater camera, you'd be better off taking photos in the ocean. The mix of waters in the lagoon causes a lot of blurriness.
Don't forget to wear a T-shirt so that you don't sunburn, and buy biodegradable suntan lotion so you don't hurt the ecosystem.