Will going a day and ONLY one day only eating fruit and drinking tons of water cleanse your body and get it ready for a diet?

Drinking tons of water isn't good for you, it can mess with your your blood chemistry balance for no sensible reason. As far as getting ready for a diet - there's no point in that either. Your body isn't listening. The most probable outcome from suddenly eating only fruit one day and drinking lots is that you get the runs. Even "going on a diet" is quite pointless. If it's your general lifestyle that's made you overweight then it will do so again once you've gone off the diet. For a sustainable weight loss what you need is a lifestyle change, with a new set of eating and exercise habits that you can hold on to for the rest of your life.