The problem is that the state doesn’t hold an easement to the beach and instead is using a simple regulation to effect eminent domain without a hearing or compensation.
It is clearly a taking.
Let the state pay for an easement and let the state stop taxing the landowner on that portion of their property that they effectively don’t own.
I've got some small personal experiences with easements in several states (none of which are CA) related to surface rights and mineral rights so here come some opinions based on this. I'm not a lawyer so don't attempt to come at it from that perspective.
circlecity is right that easements along with other roots or concepts of USA civil law often trace down from English Common Law. ECL is a wonderful body of common sense precedent. For example, the principle that a person or company is responsible to damages caused to another traces in part to a ECL legal case where a farmer built a pond on creek thus reducing the water available to a downstream neighbor's farm and thus was required to compensate the neighbor for the loss of water.
Easements though... An easement does not relinquish ownership. Typically, an easement is voluntary. The owner agrees to an easement for a defined time, type and purpose for a mutually agreeable compensation. An examples of this are pipelines whereby the surface owner and mineral rights owner are compensated typically with an initial lump sum payment then yearly payments for as long as the easement remains in effect. Key things to take away is that the owner is normally indemnified against damages cased by the easement holder and the easement holder is responsible for remedying damages caused to the owner's property.
State law gets involved in easements by defining in what types of situations, eminent domain can be used to force a property owner to grant an easement. This is where the lawyers get involved and $$$ rack up. Depending on how the state law is written, sometimes this can be a political process, can be a legal authority delegated to a state or local commission or can be a civil court process from the start. In my limited experiences, I have not encountered a situation where a political or state commission decision on property rights is not court reviewable.
One thing to keep in mind about property rights (and this varies from state to state) is that in some states an owner can loose their property ownership or rights if they do not defend them. For example, in one state I am acquainted with a situation where a neighbor obtained title to a boundary strip of land by building a new fence that encroached a distance into his neighbor's land. After a period of years, the sneaky neighbor was allowed by state law to obtain title to the strip of land. In another case, a neighbor tried to sneak some property away by beating some foot paths through the neighboring property then planned using this beneficial use to attempt to gain title. The property owner got wise to it soon enough to nip this in the bud and in fact won a legal judgment against the sneaky neighbor (a husband and wife lawyer team).
I see the subject of beach access brought up by the OP to be ripe for some serious legal bucks by both sides of the issue since the land owner has the bucks to go head to head against the State. I don't know the answers but see a lot of arguments that can and probably will be made on both sides. Buy stock in pop corn companies.