Often animals in fresh and brackish water have a higher solute concentration than the water. Thus, water tends to move into the animal's body via osmosis. This water needs to be removed from the body as urine, which also takes solutes. Consequently, ions must be removed from a dilute solution to a much higher concentration in the blood to replace those lost. This requires active transport.
Active transport consumes metabolic energy to move solutes against their concentration gradients.
It can also be noted that the greater the osmotic difference between the cell and the environment, the greater the energy needed to create a concentration gradient. This is achieved by adding the organic compound urea to the body blood until the total osmotic concentration of their blocked is greater than or equal to that of sea water. In most animals, urea is excreted in the urine, but the shark kidney actively reabsorbs this compound so that it is maintained in the blood. So, as long as the osmotic equilibrium is maintained by urea, the shark can have a low salt concentration while living in sea water.