Saltwater fish osmoregulate by transporting salt out of their blood and into the surrounding water through specialized cells in their gills. Saltwater fish also only produce a very small amount of urine in order to retain the maximum amount of water.
Osmoregulation is the process in which fish maintain a balance of their internal levels of salt and water. The adaptations saltwater fish have allow them to osmoregulate and survive in saltwater environments. Saltwater fish are hypotonic, meaning their internal salt concentration is less than their surrounding environment. For this reason, they are constantly losing water through osmosis.
Consumption of sea water would result in too much salt in the bloodstream of these fish. Osmoregulation through specialized chloride secretory cells in the gills allows for removal of the excess salt. They are able to recapture the water they are losing without absorbing any salt.
Since these fish are always losing water through osmosis, they cannot afford to excrete a large amount of urine. Saltwater fish achieve this through minimal rates of flow to their kidneys. In addition, the kidneys of saltwater fish contain very few glomeruli, which filter blood to form urine. These specialized features allow saltwater fish to maintain the proper salinity.