@browncoat - I guess it depends on what they are using them for and when they need to use them. If the weigh-in is a few days before the competition (and I believe the weigh-ins generally are held a few days before the event, depending on what it is) then there will probably be time for the drug to pass through their system before they have to actually compete. As long as they keep up their vitamins and fluid intake, they'll have no problems at all.
If they are using the diuretics to erase evidence that they've been cheating with other kinds of drugs though, then they may have to take them right up to the last minute which could cause the problems you've described. Of course, they might think the enhancing drug that they're trying to hide is worth it.
Personally, I think it's really sad that people will do that. They care more about the money and the prestige than the sport, I guess.