I do agree with you on the drinking water part. One of the good example is from the wikihow
http://www.wikihow.com/Lose-Belly-Fat-by-Drinking-Water

I believe some people who reduce the their water and sodium intake only for a temporary basis. (sodium will retain water in your body) Like for celebrities, they do it for the shooting or printadd maybe, and for those body builder they do it before a competition.

Loosing water in your body in a long run is pretty dangerous. Scientifically it called 'dehydration'. Untreated dehydration generally results in delirium, unconsciousness, swelling of the tongue and, in extreme cases, death.