salt water fish are in constant threat of dessication as a result of their hyperosmotic environment - water flows out into the environment and therefore they must constantly drink water to compensate for this "imbalance." in addition, they produce a concentrated urine. on the other hand, fresh water fish are in a hypoosmotic environment and as a result water will naturally tend to flow into the cells of the internal environment. thus, they need to "drink" less water and produce a less concentrated urine. please feel free to correct me if I made a mistake somewhere in this brief description... everything is off the top of my head.