Disputes about beach access usually come down to one question: where the private property ends. In the United States the answer is a tidal boundary that differs by state and moves over time.
The boundary is defined by tides, not by a fence
Coastal property lines are commonly described against a tidal datum such as mean high water, a level computed by averaging observed high tides over a long period.
Because it is an average of measurements, that line is a technical determination made by surveyors rather than something visible on the ground.
The wrack line of seaweed and debris is a rough visual proxy that people use, but it reflects recent tides rather than the legal datum.
States draw it in different places
The underlying principle, that tidal lands are held by the state in trust for the public, is longstanding, but its application was inherited and adapted separately by each state.
Some states place the public boundary at the mean high water line, so the wet sand is public and the dry sand above it may be private. Others recognize public rights further up the beach.
What the public may do there also varies, with walking, fishing and navigation treated differently from sitting or placing equipment in some jurisdictions.
The line moves with the beach
Shorelines accrete and erode, and the boundary generally moves with them when change is gradual. A property can gain or lose beach over decades without any transaction.
Sudden change from a storm is treated differently in many states, and placed sand from a nourishment project raises further questions about who holds the new beach.
These are genuinely contested areas of law that have been litigated repeatedly, and outcomes differ between states.
Access is a separate question from the boundary
A public right to be on the wet sand is of limited use without a lawful route to reach it, which is why access easements and public street ends matter as much as the boundary itself.
Reaching a public stretch by crossing private dry sand is where most real disputes occur, and arriving by water avoids the question entirely.
How to find out for a specific beach
State coastal management agencies and the town are the practical authorities, and many states publish access maps and plain-language explanations of local rules.
Posted signage on the ground carries weight, and because rules differ between adjacent stretches and change over time, checking locally beats generalizing from another state.