One committed OpenStreetMap coastline, cut into square cells, each marked land, subtidal or intertidal.
OpenStreetMap draws the coastline at mean high water, so land is everything above that line. Subtidal water stays wet at every state of tide. Intertidal ground is wet at high water and dry at low, taken from tidal flat, salt marsh, shoal and beach tags. A cell the coastline crosses counts as land. Cell size is set when the mask is built.