Akumal is a bay with seagrass in it, and green turtles come to eat the seagrass. That is the whole attraction and it is a good one.
Since 2016 the bay has been a federal fishing refuge with enforced rules, which exist because the turtles were being chased by several hundred unguided snorkellers a day. You enter the protected zone only with a licensed guide, wearing a life jacket, at a maximum of six swimmers per guide, for about forty-five minutes. Sunscreen is prohibited. Touching a turtle is prohibited. Entering alone is illegal and fined. The rules are a nuisance and the bay is measurably better for them.
North of the bay, Yal-Ku lagoon is where an underground river surfaces and meets the sea: brackish, calm, full of fish, and snorkellable without a boat. South of town the reef is good and quiet, and the cenotes of the Dos Ojos system are fifteen minutes up the highway.
Turtle nesting season runs roughly May to October, with hatchlings from July; the beach is patrolled and nests are marked.