A waterline is where the water level comes to on the side of a ship or boat that is in the water. A heavy laden ship's waterline will be higher up on as the weight pushes the ship deeper into the water.

Commercial ships all have a symbol called the Plimsoll Line painted on each side of the ship. The Plimsoll Line has markers which show the levels at which the ship can be loaded to depending if floating in salt water, fresh water etc. Fresh water has less boyancy that salt water therefore a ship floats lower in fresh water than it does in salt water and as a result must carry less much cargo etc.