From wikipedia - "The waterline, national Load Line or Plimsoll Line was invented by Samuel Plimsoll, to be positioned amidships, indicating the legal limit to which a ship may be loaded for specific water types and temperatures. To an observer on the ship the water appears to rise or fall against the hull. Temperature affects the level because warm water provides less buoyancy, being less dense than cold water. The salinity of the water also affects the level, fresh water being less dense than salty seawater."
Mr Samuel Plimsoll 10 February 1824 – 3 June 1898