Ships need to remain balanced or they can sink. The weight from one end of the ship to the other needs to be evenly distributed. The weight from one side of the ship to the other side also needs to be evenly distributed. The ship must always be heavy enough to stay stable, but not too heavy.
The ships weight changes as cargo is taken from ships and new cargo placed back onto ships. To make sure the ship is stable; seawater is pumped in and out of many special tanks, called ballasts tanks. In this way the weight can be evenly distributed across the ship.