Access to fresh water is a critical issue for the survival of many species, especially in desert areas. See water resources.

Even on a ship or island, there can be a "water shortage", which means a shortage of fresh water.

For fish it strongly matters how much dissolved sodium chloride the water they live in has. Most species cannot live in both fresh and salt water, though some species move between the two. Salt water fish have access to an abundance of salt, and try to get as much salt out of their body as possible, while trying to keep the water. Fresh water fish do the opposite: They have too much water, and too little salt.