contest day it wouldnt be a big deal, IMO. the day prior you will want to make sure you get enough potassium, C, etc though. before you shut off your water.
remember contest day isnt particularly the most healthy day in a BB'ers preparation phase, nor is the last week for that matter. make sure you get enough vites and mins during your depletion/repletion stage though. with the decreased variety of food its almost crucial so your system isnt too far out of whack. with all the water you are drinking, you flush electrolytes out like its cool so definitely make sure you get enough sodium early on in the week and potassium later in the week.