What's the maximum amount of water weight you can lose in one day?

Actually, unlike what the previous answer said, diuretics / dehydrating yourself and laxatives are not an effective way to lose weight. Believe me, I have way too much experience with this. If you lose a large amount of water weight, then the next time that you drink a glass of water it will stay in your system, puff you up, for days and days and days -- your body reacts to being dehydrated. Also, if you use laxatives (even just for "occasions") your body will quickly stop functioning correctly. Soon you will be constantly suffering from a whole lot of food weight - constipation that no amount of laxatives can cure.

Answer

I have lost 5 lbs in 1 day by starving myself and excersizing for an hour. I also drank only water and took ephedrine. When I woke up the next morning, I weight five lbs less.

You can also take laxitives to loose weight fast if you have an occaission.

Here's another take. I use diuretics for a medical condition. I gain water weight sometimes and I can take it off very quickly. I can lose 10 lbs a day if I don't eat sodium. This is a big no-no. I found out the hard way, when you lose water quickly you lose electrolytes. The result is the worst body cramps you have ever had. The cramps in my legs had me yelling in pain. I had to use heating pads and take a bunch of supplements and water to recover. Never take more than 5 lbs off a day. If you have 20 lbs of water weight take at least a week to get it off.