Coastal locations like marinas and public beaches often have fresh water showers for local swimmers or marina guests. Usually free, or available for use at a nominal fee (1-2$ for 5 minutes). Most times these showers operate on a timer. We don't have a shower system on pino, the space doesn't allow for it and besides, we think it would use up too much water when at anchor.
In the summer, or when in warm places, we use a pressurized sprayer as a shower. It uses little water and can be found in most hardware stores. During our north pacific ocean crossing, we use the pressure sprayer to wash dishes to further save on water. When showering on the boat we don't use soap. When water is scarce, we bathe in salt water, and rinse with fresh water.
In the winter, if we have no land access we heat up water on the stove and clean ourselves with a bucket and rag. We don't shower daily, maybe twice a week (depending on activity level).