A newborn baby can be as much as 78% water whereas a fully grown adult will be closer to 60%. In a 150 pound person, that’s over 10 gallons of water.
Regarding the capacity of the human body to consume and process water: The stomach can hold up to a gallon but the bladder must be emptied after no more than 0.2 gallons.
Most of the body's water is in the intracellular fluid (2/3 of the body's water). The other third is in the extracellular fluid (1/3 of the water).
The amount of water varies, depending on the organ. Much of the water is in blood plasma (20% of the body's total).
According to a study performed by H.H. Mitchell, published in the Journal of Biological Chemistry, the amount of water in the human heart and brain is 73%, the lungs are 83%, muscles and kidneys are 79%, the skin is 64%, and the bones are around 31%. See more at chemistry.about.com.