Desalination is also an option, it may give almost unlimited amounts of water in the coastal regions, but with quite an energy cost. Some yachts come with small desalination plants installed, particularly yachts used for long-distance sailing. The water quality tends to go bad rather fast when filling from various places and leaving it in holding tanks for weeks - with desalination one can be quite sure to get clean potable water.

We don't have a desalination plant, and we generally try to save on the water when we're on the boat. I'm usually doing the dishes and using small amounts of running water for rinsing off the soap. The used soapy rinse-water is accumulated in the sink and used for washing the dirtiest of the dishes.