Skipping food is OK for some; but skipping water can be problematic (if the weather is rather cool), or critical for one's health (if it's hot) -- loss of water is very dangerous, and when it's hot we may lose 2, 3, or even more liters of water during one such long run.
Not carrying water is OK as long as you find a way to replenish your body water reserve in any other way possible. :)