I am adding the top comment from the original article by user petethexman:

As a former Coastal Commission staff member in the 1980s, I've seen this scenario played out over and over. The so-called issue of Spanish land grants pre-existing and somehow superceding laws like the Coastal Act isn't an issue at all -- land grants occurred in the early 1800s all over California, so Martins Beach isn't any special or privileged circumstance.

It isn't just a matter of the Coastal Act. What was alluded to regarding public access along the road involves what is known as a prescriptive right. Areas with a history of public use can have the right of access through prescriptive rights regardless of the existence of Coastal Act policies. Prescriptive rights have a lengthy history -- and by lengthy, I mean going back to English common law in the 1100s. The standards for a prescriptive easement involve, among other things, that the public use be open, "notorious" (which means well known and not secretive), and continuous for a period of time. Many prescriptive rights inure in a time period as short as 5 years or as long as 20. And it isn't just the coast. If your neighbor builds a concrete block wall between your back yards and it turned out to be on your property and not his (a very common occurrence), over that period of time, if not otherwise contested or challenged, it could become his property if he was to pursue that. So the impact of these decisions is widespread not just in terms of natural resources but also public usage of trails as well.

Access to the water is a right guaranteed not just to and along the coast but to any body of water -- lake, stream, pond, whatever. Article X Section 4 of the California Constitution, enacted shortly after statehood in 1850, states in part that the public has a right to access of "the waters of the state of California," that no person, organization, corporation, or any other entity can block access to the waters of the state, and that the Legislature, in enacting future laws, must give THE most liberal application of those rights possible.

Case in point: When I lived in Lake Arrowhead in the San Bernardino Mountains in southern California, the homeowner association there blocked public access to the lake unless you were an association member/property owner on the lake. The dam was constructed beginning in 1922 and has always been in private ownership by the association. During the drought in the early- to mid-1990s, the lake level dropped nearly 20 feet, and boat owners who didn't have docks that could adjust to fluctuating water levels couldn't launch their boats. Their solution? Not take personal responsibility, certainly. No. They had the Community Services District (for water and sewage disposal) enact an ordinance that charged everyone within the District (whose boundaries far exceeded the area of the homeowners' association) higher rates to raise $60 million to subsidize the wealthy lakefront property owners who didn't have adjustable docks -- fewer than 100 property owners, in a community with over 12,000 permanent year-round residents. This money was for purchasing water from the State Water Project through convoluted three- or four-step water swapping deals with other water districts to obtain water indirectly AND was done in a manner that avoided any approvals that could be conditioned to force public access to the lake directly.

The solution to these problems? Be vigilant. Always be vigilant. The ocean is OUR ocean, and access to the beaches and water is an integral part of the culture of California. This tiny beach, away from view, away from virtually everyone except those very few lucky individuals who live there, is important. It only takes one bad precedent, one bad court ruling, to have impacts not only on this beach, not only on all California beaches, not only all bodies of water, but to any resource belonging to the people of California, whether it be a small creek in the eastern Sierra, a forest, or a desert.