Potable water can be made from sea water, through a process known as desalination. It is preferable to make potable water from fresh water reserves rather than from sea water
Sea water is heated until it boils. The salt remains in the liquid, and the steam is pure water. The steam is cooled and condensed to make potable water.
Water is put under high pressure and passed through a membrane which has tiny holes in it. The pores allow water molecules through, but prevent most ions and molecules from passing through. Reverse osmosis requires expensive membranes and also produces a large volume of waste water, so its efficiency is often quite low.