The problem I had was not staying hydrated at all. You drink tons during classroom sessions to stay awake, as stated above. Getting water in wasn't the problem, it was getting it out. It isn't always convenient.
You know you need to during class, but unless you're on the end seat, you have to walk that tiny gap between the other candidates' knees and the seats in front of them, praying you don't upset any rifles and trying not to disturb the class too much.
Or, worse, say it's just before the PT run in the morning and you are about to burst because you couldn't get your trash together in time to piss before formation. You finish the pre-run exercises feeling the urine back up so high your back teeth turn yellow, but God willing, you must piss before the run.
You have to do the whole, "Candidate So-and-so requests permission to speak to Sergeant Instructor Gunnery Sergeant So-and-so..."
You rehearse it in your head dozens of times before you get up there and sound off, but when you do it you jack it up and they send you back into formation. Then, you have to run the 3-mile with a bladder that is the size of a basketball, full of clear urine.
Running with your legs crossed is a skill.