You are asking a good question- the water content of various tissues has been measured (simply weighing and drying) way back in the 1930's.. I have a moldy book around here somewhere with the data... I did find this on wiki which is close to what I recall:



"Regarding specific tissues: Lean muscle tissue contains about 75% water by weight. Blood contains almost 70% water, body fat contains 10% water and bone has 22% water. Skin also contains much water. The human body is about 60% water in adult males and 55% in adult females."



Aside from whether or not the BMI is an accurate index of body fat composition, it's clear that increasing the amount of adipose tissue has a very small effect on body water.



But it may not be as simple as that- adipose tissue requires a blood supply as well. IIRC, losing a pound of weight corresponds to 75% fat and 25% blood/support tissue, but I can't find a reference that supports that. This abstract



http://jap.physiology.org/cgi/pdf_extract/23/5/621 [Broken]



points out that the ratio of adipose tissue to blood supply is not constant, and for obese subjects, the relative amount of blood supply is very small.