Coffee is a diuretic. Some herbal teas are diuretics. This homeopathic HCG is probably a strong diuretic as are almost all "weight loss supplements". They get you hooked with a fast weight loss in the first couple weeks, but in reality, you lose very little fat. Mostly water and muscle.
A lot of water we ingest is in our food. If you are only eating 500 calories, you aren't getting much water from your food. Drinking water helps, but most people don't drink nearly as much as they should when dieting.
Here is some info on HCG diets:
HCG Worthless as Weight-Loss Aid
Eat more real food and be more active. Short cuts will at best just lose you some money, and at worst, damage your health.