'drinking water' often doesn't mean the water a human literally drinks, but all the clean water necessary for living everyday life (bathing, flushing toilets, washing dishes, cleaning clothes, cooking etc.).
And actually it does seem like 'drinking water' is actually drinking water and not for all those other uses you mentioned. As 3.7 litres is the amount required for a human male each day.
https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-h...
> Drinking water, also known as potable water, is water that is safe to drink or use for food preparation.
40-50L per day would seem to about cover one person's essential uses of potable water.
Are you suggesting that toilets, showers, washing machines use salt water?
It would be an interesting experiment, since toilets use a great deal of water. Take a beach town, build a separate pressure water and sewer system, see if it's worth it. It's certainly more complicated than a sailboat's set up.