The problem is that desalinating water is hard; nearly desalinating it isn't much easier (it would be possible to design a bad reverse osmosis system but there isn't much market for that).
Instead if you need partially desalinated water you could get it by mixing drinking water with salt water. But there are better things to do.
Check what the toilet flushes with - I'm no yachtsman but I think it's seawater. So you can ignore that if I'm right. Cooking and drinking should be possible with less than 5 litres per person per day unless it's very hot (none of this boiling pasta in a vat business - the pan should be not much bigger than the food you're cooking, to save fuel as well).
Washing is harder to estimate. But here goes. When we had a touring caravan, a 40 litre container of water nearly did two quick showers. So there's a starting point for estimating how much you need for washing. But according to a sailor friend you wouldn't use fresh for all of that. Either get wet in the sea or with a bucket of seawater left in the sun, wash, then rinse with fresh water, which should be only a couple of litres. And that's when you need an all over wash - less than daily. After all, it's not antisocial like it would be in an office - you're all in the same boat.