Appears you are asking about water kept in storage tanks.onboard. in general, I wouldn't recommend drinking that water, but use bottled water for that purpose.
Does the yacht have a water maker? If so, just run water maker daily. If not, then you may need to refill tanks.
So the primary uses of water after that is for cleaning dishes and for personal showers. How crew takes showers greatly affects water use. If they turn on shower, get wet, and turn it off to lather up and then rinse, water can last longer than you think. If they use it like they are home then you will be refilling every three days. So it depends.
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