The low-water mark is the edge of the foreshore zone that is almost always covered by the sea, except in the event of a particular meteorological event (e.g. storm, tsunami). This border can be seen along the low tide by the accumulation of floating debris along the coast.
The low-water mark is the separation between the infralittoral level of the foreshore and the circalittoral level, always covered with water. In maritime law, the low-water mark is also used to define the base line.
Water marks made from driftwood. © Erin McKittrick, Wikimédia CC by-sa 3.0