They have a higher concentration of salts in the body tissues than the surrounding water. Conversely marine fishes have a lower concentration of salts in the body tissues than the surrounding water. The kidneys of freshwater fishes remove water and re-absorb salts and sugars. They produce large amount of very dilute urine. The kidneys of marine fishes however conserve water.
Marine fishes drink water and excrete only a small volume of very concetrated urine.
In most fishes, the gills and gut are largely responsible for the excretion of surplus salts. The Diversity of Fishes.