uh...I just got up at 7 am to guzzle a load, then slept 'till 9:30, emptied, then stopped drinking water. Yeah, i got a slight dehydration headache by the 4th/5th section, but then it could've been that I was taking teh LSAT, and it was past 4 pm w/ only a sip.
My advice would be to just stop drinking water 4 hours before the test, b/c sitting in a wet diaper or w/ a pouch of slowly cooling urine by your leg can be as distracting as the water pressure in your tummy.
TRUE Horror story:
My brother's friend took the LSAT a few years ago, and really, really had to go during the reading comp section. He couldn't hold it, but the men's room was one flight of stairs down. So he ran there, pissed, and ran back up, but on the way up, he slipped on the stairs, tumbled down, crawled back up to his test. And b/c of his slip, didn't even get to the fourth passage. And b/c he was dumb enough to not cancel, suffered a bad score.
I was so lucky that there was a bathroom on my floor, and that I only had to go during the writing section, which I personally think 35 mins is superfluous after 5 mind-grinding sections.