After years of scrounging parts, re-building pumps, and designing a watermaker system we are making water (22GPH)!
If you were to go out and buy a system like this you would be looking at eight – ten grand for an off the shelf watermaker. Water makers are pretty simple. You need to push sea water through a membrane (filter) at 800PSI.
I’ve only used the best components in my system. I rebuilt a Cat pump (270) for the hi-pressure side of the system and installed a March (AC-5C-MD) pump on the lo-pressure side. Seawater is filtered through a sea-strainer, 20 micron, and 5 micron filter before entering the lo-pressure pump which feeds the Cat pump. The Cat pump is plumbed to a Spectra hi-pressure vessel containing the membrane, then to a needle valve which allows me to adjust the pressure inside of the vessel. I’m using an old village marine 
controller which gives me all the bells & whistles of a modern system. My controller is from 1981. It controls hi and low-pressure emergency shut off, has a salinity tester, and an electronic three-way valve which I’ve decided against using. A faulty valve whether new or old can fail and contaminate your tanks. I’m installing a manual valve that will divert the water from a small tap at the galley sink to the tank. Many cruisers build similar systems without a controller, and just use the needle valve with a PSi gauge.
We have about $2,200 into rebuilding this old system and it works like a champ. I decided to run the cat pump off-engine which many discourage against but I 
built an adjustable mount where the cat pump is floating on closed-cell foam off-engine. The mount takes up the vibration from the motor as planned and works perfectly!
I’m running the system at 800 PSI and producing about 22 gallons of beautiful fresh water an hour.
Splendid is a wreck as the new galley is going in and Carla and I are working hard. In a few days I’ll have the new counter, sink, and faucet installed. After that it’s off to refrigeration 101 and box building. I’m hoping to have, by June, a new refrigeration system, galley (including stove), and water maker system all buttoned up.
I’ll need to run some charters & backpackers to afford a haul-out and Panama canal transit. After that it’s back to the Pacific to focus on traveling instead of boat stuff. At the end of the day, Splendid was a task of immense proportion but will render a home that will travel the world in simple comfort.
Peace, Love, and fresh water!
– alex