Did you know that a submarine takes in seawater from the ocean and converts it into freshwater?
Once inside, a distillation plant heats the seawater to water vapor, which removes the salts, and then cools the vapor into a collecting tank of fresh water. The distillation plant on some submarines can produce 10,000 to 40,000 gallons of fresh water per day. This water is used mainly for cooling electronic equipment and for supporting the crew (drinking, cooking, showering, brushing you teeth, etc).
After spending months underwater on a Trident submarine, I can certainly vouch for the importance of fresh water.
Pretty cool stuff!