that's an exaggeration, and a gross generalization. When performance is a priority, and you're a few lbs over where you need to be, you'll be stronger on competition day by cutting water than you would be by dieting in many cases. Particularly for someone who's very lean like me. If I want to cut to a lower weight class than what my current bodyweight is, I can't do it through diet anymore. It HAS to be water weight. Almost everyone in my sport cuts at least a few lbs of water for competitions.
Ok, so as basement said, there are plenty of variables, but there are good general guidelines available to work with, and you can adjust as you see necessary. Basement is right about the danger of cutting water if it is a lot of weight. Generally speaking, as long as you're not cutting more than 5% of your bodyweight, it's not particularly difficult or dangerous. So as a 200 lbs guy, I can cut 10 lbs over the course of a week and it not affect my health, performance, or mood adversely in any way. Once I get to about 12-13 lbs of water cutting, it starts to get difficult. Any more than that, and you approach difficulties.
I'm not going to spoon feed you on this topic. There are so many resources available just by googling. I will say that proper methods involve salt, carb, and of course water intake manipulation. The entire process should take about a week. And sauna/hot baths may be used at the end as necessary. If you read an article that advocates diuretics or wearing sweat suits/trash bags, then I would look for another resource. Those are on the unsafe side of things. I don't think I can link this site, but Chris Duffin wrote good stuff on water cutting. google kabuki water cut.