To the OP:
My short answer: allocate 1 gallon (4 liters) of
water per person per day, for all uses. Also give them each a personal and smaller
water bottle for handy drinking, and encourage drinking fresh water over other beverages. Take additional water as a reserve, just in case the voyage takes longer than expected.
More details:
I think the smart thing to do is give each person a clean one gallon plastic jug with a screw on top, and their name printed on it in permanent marker. Fill it each day with fresh water. That is their days ration for all uses of FRESH WATER, aside from
cooking. Also, encourage all crew to drink water rather than sodas or anything alcohol or anything diuretic (teas or coffee). Reason? Because the body needs water, and dehydration is bad and can be debilitating. Many of the foods eaten on a
passage will be salty processed foods.
Anticipate that bad things can happen to your
engine that may prevent using a
watermaker and that you might take longer to reach your
destination. Be prepared with enough tankage or jerry cans of fresh water to make it to the
destination with some buffer or reserve.
My Experience Anecdote: I was on a sailboat crossing from
Hawaii to
California. It was expected to take 21 days or so, because we were going to
motor when needed through the low
wind areas, and so we took plenty of
fuel. It turns out that about 3 days into the voyage we could not use the
engine at all! The trip took 30 days, and could have taken longer as we essentially sailed the whole way, aside from being becalmed for several days. After 20 days of sailing, and still not knowing how much longer till landfall (10-15 more days?), we began to ration the water. I learned about real thirst then, and eating salty canned foods and crackers, which was was all we had left in the
galley stores, served to worsen this. Also, the water in the
boat tank was very foul as we got to the bottom of the tank. It was so disgusting that no one wanted to drink it at that point. It had heavy green algae and was the consistency of pea soup.
My water lessons learned?
Have more water than you think you will drink for a voyage that could take longer than average for that
route and that
boat. You should drink more water than you normally do in a city, because your exposure to the sun and
wind and physical activity may make you more thirsty. Avoid beverages that cause more thirst. Have backup water in separate
tanks or cans or jugs. Have an easy way to allocate fair rations each day at a set time (I suggest morning) for everyone in the crew. Use identification on personal water jugs. Anticipate that mechanical systems (engine or watermaker) CAN fail. Have more water, rather than spare
watermaker parts as a backup. It may be other systems (engine) which may prevent the use of a watermaker if your boat has one.