A solar still is a device for turning unpotable water into potable, through the power of the sun.

You can improvise one on land quite simply*, but they are also available to buy, particularly for use at sea, where the improvised method will not work.

A typical still consists of an inflatable ring (in black to absorb the sun's energy), and a cone-shaped cover of clear plastic. The whole unit floats in the sea, tethered to your life raft.

          .
         / \
        / c \
       /     \
      /       \
     /         \
    /           \
   /             \
___|   |_____|   |___________Sea Level
   | d |  b  | d |
   ++ ++-+ +-+---+
    |e|  | |
    | |  |a|
    | |
  +-+ +-+
  |_____|
  |  f  |
  +-----+

Sea water can enter through the hole a and makes a pool b in the centre of the still. Sunlight evaporates the water, which condenses on the plastic cover c.

The condensed water - which is now free from salt, runs down the sloping sides of the cover to the outer ring d, exits through hole e, and collects in the container f. When you need a drink, simply fish the container out of the sea, and hopefully it will contain at least a little drinking water.

A possible problem with the still is that if the sea is too rough, then the saltwater may splash into, and contaminate, the drinking water. However, it has saved many lives, and should be a staple in the kit of the adventurous sea-farer.