There's nothing like a long hot shower, whatever time of day you take it. On the JOIDES Resolution, there's always plenty of hot water. In fact, about fifty tons (15,500 gallons) of water (~135 gallons per person) are used each day. (Average indoor domestic water use in the United States ranges between 60 and 100 gallons/day.)
The water tanks only hold 150 tons (46,500 gallons)—a three day supply—which means that almost as soon as the ship leaves port it must begin producing its own water supply. Water is created by desalinization, an energy intensive process that removes salt from seawater.
Two different types of evaporator desalinization processes create between 9 and 85 tons of water each day. For both, seawater is pumped into a tank, where it passes through a steel mesh and a 30 micron filter to remove any particulate matter.
In the first method, the water is fed into another tank where the pressure has been lowered by creating a partial vacuum. This allows the water to boil at 165 degrees F. The water vapor is collected, condensed, and stored in a water tank. The salty brine is returned to the sea.
The second method has more steps, several of which exchange heat between outgoing warm fresh water and incoming cool seawater to increase thermal efficiency. After passing through a heat exchanger, water is sent to a reservoir with electric heaters. From there, a recirculation pump pushes the water into a sprayer located above heating coils and the reservoir. Water vapor created by the spraying passes through a demister and into a condenser. Liquid water falls back into the reservoir. The compressor increases the temperature and pressure of the water vapor which flows through the coils onto which water is sprayed to cause evaporation. The compressed vapor cools into water and passes through a heat exchanger with the incoming sea water and into the fresh water holding tanks.
When needed, water from the holding tanks passes into a pressurized tank for shipboard use. There, the water pressure is monitored. Below a certain pressure (e.g. 30-32 psi), water from the holding tanks begins to flow in. When a maximum amount of pressure is reached (~60 psi), the pumps stop and water ceases to flow into the pressurized tank. Just before passing into the pipes for consumption, the water is sterilized with UV light.
The engineer tries to make sure the water tanks are full before entering port. Once the ship is within thirty miles of a coast, the possibility of contaminants increases and Coast Guard regulations require desalinization to stop. A long port call means having to buy water at the dock, and in some ports water can be more expensive than fuel!