Going up north (WA) later this year for two weeks, 2 adults 2 kids, towing a camper, once we leave Geraldton wish to be water independent, does anyone have a formulae or experince with water consumption, ie how much do I take per day? Will be camping on the coast so an abundant supply of salt water for dish washing/ washing etc OK, but cooking, drinking etc.
Apologies in advance for the gross assumptions... 
Depending on what version of the literature (or propaganda) you believe, you are looking at between 1-3L of liquid water (as opposed to what you get from food etc.) per adult person per day. Some literature claims 3L is the minimum for a sedentary adult (which I find very hard to believe indeed) but as a guide 4wd'ers apparently budget 3 L per person and that seems to be a pretty safe number as your daily usage will likely be around 2L per day.
Don't forget depending on how remote you will be, you should always have a backup supply for everyone too.
If you are near salt water, away for a while and have access to fire wood you can purify the salt water without too much trouble - just don't let the greenies see you... 
that's my 0.02L anyway...
see:
www.biopsychiatry.com/h20.html
www.snopes.com/medical/myths/8glasses.asp
www.exploroz.com/Forum/Topic/37055/Desert_Drinking_Water_Consumption.aspx
www.armystudyguide.com/content/army_board_study_guide_topics/desert_operations/water-usage-in-desert-ope.shtml
I would factor heat in to the equations.
I drank 12 l of water in one 12 hour shift once... I was working outside with no shade, and it did get to >55 deg C.
I would take as much as you can carry and budget from there. After a certain point you have enough, and the rest just adds to comfort. Obviously you need enough to drink, but it's also nice to be able to rinse in fresh water after a salt water shower - even if only every second day. (Chicks dig that stuff). Also washing up etc all nicer in fresh if you have enough.
P.s. if you have a camper and a tank with a pump don't be surprised if it goes quick! I use 20l jerries and it's easy to monitor usage and don't seem to use so much when not so convenient to use. In my exp if I have it on demand or easy to use, or if I think I have heaps at the end and start letting the fam have hot showers etc it goes SO quick!
Does anyone know if you can get mini desalinators for camping?
You can get units for boats
I expect they would be expensive, but if you plan on being "away from it all" for an extended period they could be useful.
