Why does a ship float lower in the water when it goes from sea water into a freshwater estuary?

Saltwater is denser than freshwater:When salt is dissolved in water, as it is in ocean water, that dissolved salt adds to the mass of the water and makes the water denser than it would be without salt.

Because objects float better on a dense surface, they float better on salt water than fresh water. The denser the salt water, the easier it is for objects to float on top of it.

source:infoplease