hi, Mikel,

a empty ship or a ship with a heavy load on deck but less cargo in the holds may take on weight stored far down in the hull, called ballast (not really a cargo) to lower its point of gravity, i.e. to reduce the risk of capsizing in heavy seas.
Nowadays most ships can take on ballast in form of water in special ballast tanks.

Sailing boats may have a keel ballast (e.g., lead) to keep them upright when under sail.

greetings, Urs