Dear Lai Chieh Ying,
We don’t carry much fresh water on the boat, but we have a desalinator and a backup, to make fresh water from the sea water. The unit is called a PowerSurvivor 40 (you can find it on the web), and it will take in sea water, and make 1.4 gallons of freshwater per hour. This what I use for cooking, cleaning, drinking. It’s called a reverse osmosis desalinator, and essentially forces the sea water through a membrane like a fine filter which takes out the salt molecules. We then have a plastic bag called a Solar Shower, and I can fill the bag with fresh water, and put it out in the sun (if the sun is shining), and it will heat up the water. It has a short hose with a screw nozzle. Sometimes I’ll lather up in saltwater first, because the freshwater is so precious. But to do that, you have to try different soaps, to find out which will work in saltwater. (We’ve found that dishwashing liquid works well!) If I’m going to wash my hair, usually I will get a bucket of saltwater, and stick my head in it upside down, holding my breath, and then lather in the shampoo, and then rinse in freshwater. It’s all a bit complicated, and every sailor looks forward to a nice long hot freshwater shower when we get to the finish!

Question from LAI CHIEH YING, Miaoli, Taiwan