Water ballast : Water confined in specially constructed compartments in a vessel's hold, to serve as ballast.

Example:


Taxonomy
ballast, boat, keel, water ballast, hull, tank, ballast tank, Sailing ballast, high density, crew, free encyclopedia, Ballast water, windward side, canting keel, sailing vessels, valve, lee side, sail.

Defination from the web:
ballast clear of the water, at which point it is obvious that it does provide righting moment. ... A trick commonly used on boats with water ballast is to link port and starboard tanks with a valved pipe. ...