There have been a bunch of threads on this subject which you should look over as the discussions have been rather extensive. The bottom line is that it depends on what you need and what resources you have to support it. It also depends on what you mean by manual. If you mean one of the hand pumped units like
Katadyn sells it is unlikely to meet your needs. If you mean by manual that you have to turn the switches and valves instead of having a computer manage them you have a huge number of units to choose from. Basically you have three
power choices,
engine driven, 12V DC or 120/230V AC. If you have a genset you can pretty easily run a 120V/230V (depends on which you have) unit that can produce 40 gallons per hour. If you have lots of
solar and are in a sunny climate a 12V unit might do but they typically produce
water at a much lower rate. Given that you need to support 6 people you might want to consider one of the higher capacity units as you will spend a lot of time babysitting your
water maker if you don't(the disadvantage of a manual unit).
Engine driven units can produce at the same rates as the AC units. They are convienient if you typically use your main engines to charge the
batteries. If you have a 12v unit and do this you'll have to run the engines way too long as you will be using some of the
power used for
charging your
batteries for making water as opposed to
charging your batteries. The Engine driven units can put out a lot of water in a relatively short time and you give your batteries a boost at the same time. I have a
Katadyn 160 which produces about 6.7 GPH. It was in the
boat when I bought it, but my experience is that 6.7 GPH doesn't cut it for me. With 2 people on board it's marginal, but when I have guests I have to run it way too often and way too long. We have a
washing machine on board but cant use it because it uses too much water. I simply don't want to spend my life babysitting a
water maker. I would like a 40 GPH AC unit. Interestingly one can be had for almost the same
price as a Katadyn 160.
Like I said , look at some of the old threads. You'll learn a lot and probably be better able to judge your needs and get a water maker that fits those needs.