I have seen showers used by hire boats….especially the ones with more beds than hot water capacity!! Sort of makes sense if you need to get 8 people through a shower of a morning.
Also useful for those with only a back cabin or people like myself when I was doing some bathroom refurbishment….especially when my marina at the time didn’t have such niceties….but they are expensive to maintain especially when people don’t look after them…even when we did move to a marina with all mod cons it amazed me that moorers used to break the unbreakable and block drains for a pastime.