So you live by the coast and you are worried about fresh water when shtf. Well look to the ocean for your survival. You will need a relatively thin metal pan and material enough to make a tepee above said pan. This tepee should have edges well defined and under the edges in containers in holes if need be, under the tepee place cup, pan or what you have. Let the pan heat the water heat slowly to a mild speed. The water fed into the cups under the edges of the tepees should be pooled in a bucket. The bucket can then be put through a sand charcoal filter then boiled for about ten minutes.

This is obviously a slow process, but if you are in a secure location you should be able to get ahead of demand.