Marine fish, living in a hypertonic environment, suffer from a shortage of available water. They drink seawater and extract the water from the salt solution for their metabolic needs. However, they are left with excess salts, which must then be excreted. This is largely accomplished through glands in the gills that excrete NaCl. A variety of ATPases participate in the reaction, which is highly endergonic. In some marine fish, special rectal glands excrete excess salts into the terminus of the digestive tract, where they pass into the external environment.
Since freshwater fish are surrounded by a hypotonic environment, their problem is quite different. They must get rid of the fresh water that is constantly moving into their bodies. To drink water would be counterproductive, and so they never drink. Instead, the little salt that is present in the water around them is drawn in across the gills. This process also requires energy and involves the participation of ATP-splitting enzymes in glands within the gill.