The laws of physics are working against you. The formula for required
pump horsepower is GPM*PSI/1460. a single 21" membrane needs a minimum
of 1 GPM at 800 PSI which means you need about 1/2 HP. That is close to
40 amps at 12V. You would need a 100 amp alternator to charge the
battery bank and make water at the same time.

My future boat is small... I must mantain a lower profile for power and
water use...
But you have also one Jacuzzi in your boat? :-)
What about a windmill power generator or towed generator to refill
batteries?
The windmill is easy to make and can be in the 1 meter diameter range.
I don't have data for the towed generator, but must be a fairly easy job...


I would look at the Waterlog tow behind watermaker if I were you. About
980 Euros. Not as convenient as a traditional watermaker but you can't
build one for that price, it can put out about 20 to 28 gallons in a
full day of towing and it uses no electrical power.
http://www.yach****ermaker.com/

I have seen it, but my lathe collect dust... I tink that you understand me
:-)

Paolo