After the diet, the water immediately returns. The original fat lost returns, along with more fat. The muscle is lost forever unless it is rebuilt by exercise. When muscle is lost, the results are the same for your body as they would be for your car. Throw out part of the engine of your car and see if it will function properly.
Muscle is the engine of the body and burns fat for fuel.
Your weight loss goal should be to have fat loss only. This will not happen if the body is not serviced properly through healthy eating and regular physical activity. Healthy eating includes following the recommendations made by the USDA Food Guide Pyramid. Avoid excess alcohol, sugar, sweets, and fats. Be active every day.