Until now natural=coastline is the only tag defining the coast, and is used to map the Mean High Water Spring line. There is no structurally equivalent for the Mean Low Water Springs line, nor for the neap tides.
This proposal tries to solve most problems derived from it:
The spring tides (when the range is greatest) define the tidal range of the coast. natural=coastline defines the Mean High Water Springs, and is the main line used for the coast in OpenStreetMap. natural=mean_low_water_springs defines the Mean Low Water Springs, and most of the maritime boundaries derive from it. Between these two lines is the intertidal zone, rich in different map features, such as beaches, wetlands and bare rock, which should be maped as multipolygon relations.