The law in Mexico reserves the entire Mexican coastline as a Federal Zone. This zone extends landward 66 feet, 10 inches from the high tide mark. This swath of Federal land is open to the public for their use and enjoyment, 24 hours a day, 7 days a week, 365 days a year, SO LONG AS YOU HAVE LEGAL ACCESS TO THE BEACH.
More about legal access a little later.
The exception to this rule are Navy installations, Administación Portuaria Integral (API) installations, Caleta Marina, Fonatur Marina, the piers at Punta Langosta, Puerto Maya and the International Pier, the municipal piers and their surrounding areas, municipal, state, and national parks (like Chankanaab and the Eco-park at Punta Sur), and sections of the beach that the government has granted a special concession to the adjoining property owner. These special concessions are usually granted to Hotels, Condo projects, Beach Clubs, Bars and Restaurants by ZOFEMAT, the federal agency in charge of the Federal Zone.
Typically, a homeowner with beach-front property does not have a concession, but there are a few cases on the island where they do. However, wherever concessions have been granted, the public has only the right of ACCESS through the concession, and NOT the USE of the beach of the concession without express permission of the concession holder. What all this means is, most of the beaches in Cozumel are open to the public…IF you can find a legal access to the beach and IF it is not under concession or restricted access due to one of the reasons cited above.
You can’t just walk through someone’s property to get to the beach (that is trespassing, a serious crime in Mexico); there has to be some formal, open, public easement to walk or drive through.