I had a girlfriend who was chronically underhydrated, and for her a drinking regimen was actually useful. (Unfortunately, she learned this after we broke up... Her dehydration actually reduced her sex drive, because: low lubrication.)

I only know one example, however; for most of us drinking enough is pretty easy.

As for the other end: one particularly hot summer at Pennsic (two-week camping event), we had people who went to the emergency room for low salt levels due to drinking too much water without electrolytes. Turns out that it IS what your body craves - just not plants, so much.