Aldosterone, produced by the adrenal cortex, is one of the primary hormones responsible for the maintenance of water and concentration of sodium, chloride, potassium and magnesium (electrolytes) in the blood, in the cell and in the area between the cells.  When the body is working correctly, the fluid balance and salt concentration is very similar to sea water.

The production of aldosterone is linked to the production of cortisol.  If the production of cortisol is down, then the production of aldosterone is down.  Since most of the actions in the body are related to the proper concentration of electrolytes and their ability to carry electrical charges between the cells, keeping the proper balance is crucial.

If you have low cortisol production from the adrenals, then you have lowered production of aldosterone.  Lowered production of aldosterone means that the sodium excreted through the kidneys is not replaced as it should be.  As the amount of sodium falls, you find yourself craving salt.  If the sodium is not replaced, then the cells start to lose their sodium and the water is depleted as well.  This leads to dehydration.  When you try to hydrate yourself by just drinking water, it often dilutes the amount of sodium even more, which just worsens the problem.