Hmm. Maybe it's just my metabolism but without water there is no physical functionality.
Hell, I can't get through an hour at the gym without at least a liter.
It would seem that to get through a full fight you would need to be well hydrated...and without the benefit of a camel's hump that means you're going to have fluid stored (presumably in excess of what you'd sweat out). From memory, just hitting the bag and working the pads for 12 straight rounds works up quite a thirst - let alone competing with a live body.
Never had the experience of being nervous to the point where it brought on the "need to go". Maybe because the need arises so frequently anyway on its own.