Derives from 10m ocean coastlines. Ocean polygon is the same as Bathymetry 0 (sea level).
(below) Yucatan peninsula, Cuba, and Hispaniola.
Caspian’s Garabogaz Aylagy and other “lagoons” might better belong in the lake theme than oceans.
This theme (and it’s inverse cousin, Land) are used to clip the raster imagery.