Americans are estimated to be 5-10% dehydrated, but too much water can be just as fatal as dehydration. I would suspect that this estimate is low base don my trip to the grocery store. Everyone I saw was loading their cart full of soda and coffee. Both of these may contain caffeine which will worsen the dehydration by increasing urine output (makes you pee). A balance is key.
Our bodies are made primarily of water. Approximately 60% of your body is water and it is essential for life. To operate efficiently, transport nutrients and electrolytes, lubricates our joints, transports nutrients, wastes and oxygen in our blood, regulate your body temperature, excrete wastes, and to properly digest and absorb food, you must have proper hydration. Water lubricates our joints, Water is required to keep your body cool and to regulate waste. If you are dehydrated, you body will hold onto water and slowly reduce its ability to perform these functions correctly.
- Water promotes weight loss. One study indicated that 2 cups of water prior to each of the three main daily meals may increase weight loss when combined with a hypocaloric diet, as compared to a hypocaloric diet alone.  Another study showed a decrease in in the subjective feeling of hunger with hydration before meals. Water intake may promote a feeling of fullness or hydration my reduce the feeling of hunger. Either way, more studies are needed.
- Thermogenesis. Water has been shown to increase energy expenditure. One study showed adding a little over 500 ml of water to your diet increased metabolic rate by 30%. That being said, it is not 30% per 500ml. Further intake is not proven to cause any additional benefit.
- Water before meals: As little as 500 ml of water 30-40 minutes before meals has been shown to reduce the calories eaten. On study looked at energy intake in elderly obese patients and found that if they drank water prior to meals that they ate 13% fewer calories. Another Study by Van looked at water pre-meal in older vs younger adults and found that water did not decrease calorie intake in younger adults, but it did in older . I wonder what they would find if a little Metamucil or Citrucel was added to the water with each meal. On last study that looked at primal water found that subjects with 500 ml pre-meal water showed 4.4 pounds higher weight loss over 12 weeks.
I would argue that water is the most important nutrient for human since our bodies consist of such a high level of it. Drink it cold because you body has to spend energy to warm it up. Even if hydration is proven to not help weight loss with further research, it can do no harm for most Americans to have 3-6 extra 12 oz. glasses of water per day. You must consult a physician for advice because patient with heart and kidney failure should avoid extra water.
Recommendation: Drink 90-128 oz of water per day and at least 500ml before each meal.