As we all pretty much enter the hotter parts of the year dehydration becomes a big deal, this is most likely what had me so sick for a week thinking something was massively wrong and that I might need to go to the hospital and kept me from working out due to extreme weakness and light headedness as well as nausea. After forcing liquids for a few days I feel better. I know a lot of people know this stuff by heart but I thought I did and was surprised how little liquid you need to be down to feel the symptoms so here is some info I found....
http://www.symptomsofdehydration.com/
The Causes of Dehydration
There are many things that can cause dehydration, the most common are vomiting, diarrhea, blood loss, malnutrition, (read more causes at linked site)
The Signs and Symptoms of Dehydration
(read more symptoms at linked site)
•Extreme fatigue
•Muscle cramps
•Headaches
•Nausea
•Tingling of the limbs
(read more of article at linked site)
Only YOU can know how much water YOU need to be at your best. Thats right, WATER. Not soda, not juice, not sugar-drinks. Pay attention to your fluid loss and take special care to replenish it as it is being lost. By the time you feel thirsty you are already dehydrated - you want to avoid becoming thirsty in the first place. Pay attention to the color of your urine, dark urine is usually an indicator that you are dehydrated. Drink more water, especially infants, children and the elderly.
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((edited by admin to remove bulk of text due to copyright. Please read entire article linked above))

Even whole foods contain water. Obviously if you restrict calories or sugar then you'll want to choose your fluid accordingly. But technically it all counts toward your daily quota of fluids.