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This thread caught my eye because I was contemplating building one myself. I have not seen or heard of any small enough for a boat. They are quite simple to design (even the large industrial units have three moving parts!).
Once the vacuum pump has initially evacuated the shell of air, introducing water into that shell will immediately produce some vapor to try to equalize the pressure (nature abhors a vacuum). This provides the starting steam. By running cooling water through the condenser portion, you will condense some of this vapor back into clean, desalinated water. The process will be very slow at this point, you need to add heat to the feed water. Almost any heat source will work (ships do, in fact, use diesel engine jacket water for this purpose, or steam from a boiler, or even electric if necessary). The more heat you add, the more water you'll produce. The heat you can recover within the cycle, the more efficient your system will be!
This all applies to most common flash evaporators used in industry. A type that I've not directly dealt with, but to me seems more user friendly, efficient, and better suited for boats is the vapor compression type. It uses a similar cycle to a refrigerator to recycle energy while using very little. If you find any commercially available evaps for boats, let me know! Good luck!
Here's a site I found after a quick search with some good diagrams: http://maximevaporators.com/PageDisplay.asp?p1=6097