it does not alter normal bodily function.
Actually in large enough doses, it can. Water intoxication can occur, and that yields many risk factors and symptoms. It also can cause Hyponatremia, which isn't related to the intoxication, but it's caused by water, due to therethen being an imbalance of electrolytes (too many), and too little sodium in your body.
Not to mention urination is obviously part of your normal bodily function, but a side effect of drinking water is increased urination, which is altering your body (e.g. urinating more frequently than you would if you didn't drink water [i.e. got it indirectly through milk, juices, soda, etc.] or drank less).
There's also people who are proponents of the thought that drinking cold water raises metabolism, because the body then must work harder to regulate it's temperature.
It's fair to say water alters bodily function, but I agree with you on the statement (by my definition, likely yours, the WHO) that water wouldn't be a drug. However, SuperBowser seems to have a different definition of the word, which is completley fine.
Sidenote:who's that in your avy?
Ben Gibbard (lead singer of Death Cab and The Postal Service, though he's done work with other varoius indie bands)