When I first road-tripped down this Mexico coast in the early 1980s, I stumbled across Yal Ku by accident. In those days, there was a hand-painted sign pointing to the lagoon. No gate, no entrance fees, no people. It was the best snorkeling experience I had ever had, turning me from a casual snorkeler into a hard core snorkeler. Perfect visibility and varying depths (4' to 15'). Schools of colorful parrotfish, with occasional sea turtles and menacing gar.
Ah, progress. Now Yal Ku is gated off, with a 75 peso admission and a few more bucks if you need gear. The grounds are populated with engaging bronze sculptures, and offer lockers and a restaurant. There are more access points to the lagoon, although some need some maintenance (slippery algae-covered ladders, stairways with nails sticking out).
But the lagoon is still clean and very active with critters. The area nearest to the Gulf is for strong swimmers only. During our recent visit, the waves were really rocking there. But the rest of the lagoon is flat and calm.
There are a few underwater "caves" and channels for the daring snorkeler. At one spot near the western shore, we found a huge school of minnows or sardines who swirled around us and followed our arm motions as symphony musicians follow a conductor. Fantastic.
It got a little loud when a busload of screaming Texas frat boys arrived, taking over the western section of the lagoon, but Yal Ku is still a great snorkeling experience for the beginner or expert snorkeler. Get there early, but not too early...it says it opens at 8 a.m., but it really opens at 9:00 a.m. Free parking for about 12 cars inside the park.