What marks the boundary line between the seashore and upland estate?

The boundary line between the seashore and the upland estate is marked by the line of mean high tide. This designation is significant because it represents the average point where the tide reaches its highest point, which is essential for determining property rights and ownership.

In coastal land law, the area seaward of the mean high tide line is typically considered public land, while the area landward belongs to private property owners. This delineation helps in managing coastal resources and protecting property rights. The line of mean high tide is a stable reference that accounts for the periodic changes in water levels due to tides, making it a reliable marker for establishing boundaries in land surveying.

The other options do not provide the same clarity or stability as the line of mean high tide. The low water mark fluctuates and does not effectively define the boundary since it can shift significantly with tidal changes. Private property markers may vary from one piece of land to another and do not apply universally along coastlines. Similarly, a state-established boundary could be defined for specific regulatory purposes but would not inherently reflect the natural characteristics of the coastline like the mean high tide line does.