Why fishes are not salty though they live in salty sea water?

Since saltwater fish are surrounded by the ocean which
contains high concentrations of solute as compared to inside their own tissue, their
bodies tend to lose water to the environment via their gills due to osmosis. To prevent
dehydration, they must constantly replenish their water supply thus they must constantly
take water in by drinking it. However, their kidneys and specialized cells in the gills
must remove the excess salt to prevent them from dehydrating.