A ship sitting in the middle of the ocean can run out of water. While I was in we used a distiller to make fresh water. Steam was used to evaporate the water and leave the salt behind. Some of the newer ships might use reverse osmosis now. When everything was working well, we could all pretty much take a shower and had no problem with water being available.

Fresh water was used for many different reasons onboard ship. We needed it for boiler water to make steam to make more fresh water. Drinking water, washing water, laundry, galley, and chilled water for the electronics. So if there was ever a problem with the distillers we had to be careful of our water usage. Normally the first things were to switch the toilets over to saltwater flushing. Then cut off the drinking fountains around the ship. Cut off the extra deep sinks around the ship used for mop water.

Showering was then only allowed during certain time frames. This is where the term water hours comes from. Restricting the times when showers could be taken was a big help to saving water. If that did not work, the engineers would shut off the water heaters. We still had hot water in our berthing compartments, one bonus for being engineers. If water usage still did not taper off the showers all over the ship were shut off, including the chiefs and officers.

Going without a shower for 2 or 3 days was just so nasty. The poor deck division would get real ripe after a day, then engineering would start to stink in about a day and half, and then maybe supply would start to stink. Soon everyone on the ship started to get ripe.

So, this only made the enginemen work harder getting the evaps working right. The engineers could rig showers in the engine rooms and use those. But it was dangerous to do because you didn’t want to get caught.
If it rained people would stand outside just to get a little rinse down. I even saw a couple brave/stupid people wash their hair during a heavy rain storm. They got lucky and had enough time to rinse their hair out before we passed the storm. If memory serves me the longest water hours time period we had was 3 weeks, with no showers for 5 days. Guys with electric razors made a decent little business those 5 days as well.