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There is a federal law passed in the late 1800's ... Neither the Herald, the US Park Service, or the Bureau of Land Management care to respond to my inquiries. There is also case law in California that says the same thing.

Most, if not all, of the US federal code is available online. Google is your friend. ;)

Speaking of Google, using the search: US federal law oceans public beaches

I found the Surfrider's Foundation interpretations (admittedly biased) with handy summary of the laws by state. California's interpretation of the public trust doctrine means the public beach is the wet area below the average high water mark. Bring your wading boots, and note that unlike the beach, beach access may not be public.

A little more searching finds that in 2007, prior California state laws requiring beach property owners to remove barriers to public access was overturned. Obviously, this current case touches on whether or not barriers may be retained. The California decision is: 19 T-WR, L.L.C. v. California Coastal Com., 152 Cal. App. 4th 770 (2007), as referenced by a wordy summary of Florida's position on beach ownership

Oregon has another definition. This differs from the California situation, but that's my point: the underlying federal Public Trust Doctrine gives navigable waters to the public and non-navigable areas to private control (if someone buys them), but states are allowed to define navigable waters in their jurisdictions. So any blanket federal laws from the 1800s are applied differently by state.

(A side note: navigable waters may not be the relevant term. Navigable waters tend to refer to riparian legal situations, while beach public/private situations are simply under "public trust." But I'm just nosing into this for the first time.)

Beach ownership law has moved on since the 1800s with two Supreme Court cases, numerous state laws, and plenty of lawsuits. See paragraph 7 in that reference. A decision that would probably weigh in favor of rich douchebags is that there's a legal precedent (Nollan v. California Coastal Commission) for requiring reimbursement of property owners when forcing public access...but not always. Decades of court fights since Nollan v. CCC have examined ways of getting public access without triggering reimbursement, usually based on "legitimately advancing the state's interest without reducing property values."

Short version is:

1) There's not a monolithic federal law defining public beaches, but rather a vague legal principle implemented somewhat differently between states. It tends to be "below the high water mark."

2) Rich douchebags have precedent for getting reimbursed if they have to give public access to their beach, though it is far from guaranteed.