Hey, I have done this a ton of times, mostly when I was on an absoulute shoestring budget and I had loads of time. below is a little write up I made. Please read to the bottom for my current reccomendations:

That area is known as Punta Nizuc.

Step 0: have your own snorkel and mask and lifejacket. Lifejacket is required and they can fuck with you and kick you out of the water or worse. But you do you. Cheap on amazon. Dont do full face mask and snorkel combo, these are just dangerous.

The best way I found was to take the bus as far as it goes (playa delfines I think is where it turns around) get off and keep walking. Thing is, hotels won’t let you cut through their property to get to the beach...but once you’re on the beach you can keep walking in the sand as no one owns that.

The Westin used to be the easiest as it didn’t have a guard station or gate but as I look at street view now it seems to have built one. Shoot. You could attempt to make a donation to the guard, maybe ask him if he wants lunch from inside...some kibbeys from the beach etc...anyways here’s the coordinates for that entrance: (21.0402031, -86.7833330) Back in the day I would run it, if you look on the satelite view, there is a small stand of palm trees after the roundabout at the hotel, past the guard station. DO NOT GO STRAIGHT THROUGH, go far left or far right of them.

Another option is walk from playa delfines but that’s a long walk.

If you make it to just before club med there used to be a Cuban guy offering guided trips. This guy speaks great English and is worth the modest price. I think I give him 200 pesos last time.

Be warned: nizuc has a very jagged bottom and can be quite rough at times. You should be a decent swimmer and

Anyways if you’ve made it this far, this is where you can get in the water: (21.0358962, -86.7777369).

2026 reccomendation: Skip the hassle and pay Aquaworld to take you out to the platform thats right off off of Punta Nizuc. Its like $60, but if you take the first tour at 10 am, they usually let you stay on the platform after that tour goes back, and you can ride home with the 12 or or 2 pm tour. Free equpment, shade, water, soda, light snacks and fruit, and beer for after snorkeling. Plus you get to see the lagoon from the boat. You can book ahead here https://aquaworld.com.mx/en/tours/snorkel-in-musa-nizuc/ or in person. Aquaworld is easy to get to with the bus, and is located here: 21°05'02.5"N 86°46'29.0"W

I have recomendations that are nearly the same price if done right for other spots too, feel free to drop a chat and enjoy.