One way is to collect rain. Another way is to erect cone shaped metal objects on which water will condense at night and run into a container. There probably are more esoteric systems but this should get you sarted.

If you don't have any machines or modern day tecnology and you DO have a salty sea how will you get clean water?

WEll, wait for rain, and collect rainwater.

In Navy survival school, we learned how to collect dew.

How do you collect rain water

In a bucket. A tarp. Fruit jars. Sheets of plastic. It depends upon how much you want.

Anything from cans to sheets of tin or plastic.

a bucket 1/3 full of s.w. with a jar in the center place a tarp (or clear plastic)over the top with a rock in the middle to form an inverted cone to direct the condensate run to the center and then fall into the jar.