Dehydration and severe caloric deficiencies will definitely drop the weight, but be ready to suffer the consequences. Your body will panic thinking it is being starved and when you do decide to eat again, the pounds will pack on.
Just recently a study revealed that diets just don't work long term and serve as a predictor of long term weight GAIN.
Diets just make you fatter | The Daily Telegraph
You must read the book "Burn the Fat, Feed the Muscle". It's by far the best text I have read regarding the healthy way of lowering body fat while maintaining muscle mass.