If you are going to run out of drinking water but have oceans [ literally !] of saltwater readily available, then a reverse osmosis pump is an obvious solution [ pun!] My need is for potable water on long distance kayak trips where fresh and safe water at landing spots along the coast cannot be relied upon [ we are talking Australian conditions ]. I carry a PUR 45 manual pump which cost a lot of money but it gives me that survival edge. As long as I look after the machine, it will look after me, and provide me with as much drinking water from the ocean as I can drink and/or carry. The pump weighs about as much as about three litres of water, and is carried in my kayak, so weight is not really an issue.
John Dowd in his book: Manual on Long Distance Seakayaking has pictures and description of a still which he used over a wood fire to distill fresh water from salt. But that was some time ago, and I think he would probably stretch to a reverse osmosis pump if he did the same trip [ South America ] again.
But obviously these remarks relate to folks who have a need for fresh water on coastal environments.
And has been mentioned already, you have to adapt your survival techniques to your terrain. In some parts of Australia, for example, survival quantities of water can be gained by covering the branches of certain trees with plastic sheeting and gathering the respiration [ in strong sun! ].
And certainly, in this day and age, a high-end fresh water filter system is vital to filter out giardia and other nasties from soak, river, or bog water.
Boiling water is fine, but if you're trying to conserve stove fuel and haven't got natural fuel then you might not have this luxury.
Regards, PeterR
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"Serve in Love; live by Faith"