Dehydration

H2O, or Thirst, is a derived statistic in Fallout: New Vegas in Hardcore Mode.

Characteristics
Thirst is affected by certain consumables and other things. Your endurance will decrease as you become dehydrated, which get worse over time. If you do not drink a suitable drink in 3 days, you will die.

Be careful what you drink to quench your thirst, as not all beverages are created equal. Some drinks (Nuka-Cola and its variants, for example) will actually dehydrate the Courier further, rather than replenish his fluids.