The Rambam [Shabbos 9,3] writes that there is no Bishul on items
which can be eaten raw. The Magid Mishneh traces the source of this
halacha to our gemorah where there is a machlokes if there exists bishul
by oil, which implies that everyone would agree that if raw oil is
usable, there would be no chiyuv of bishul.
I can not understand this. The gemorah is explicit that any machlokes regarding oil is in contrast to the halacha of water in which
everyone agrees that bishul exists. Certainly, water is an item which is consumed in its raw state. Thus, rather than being a source for the Rambam's halacha, our gemorah seems to be evidence to the contrary!?!Excellent question! The Iglei Tal (Meleches Ofeh #19) discusses this question at length and it is well worth seeing the entire text. In section #13 he offers this answer:
Oil is not affected by Bishul and is no different to us hot or cold. Water is also unaffected by Bishul. However, hot water is a pleasant drink unlike cold water. Therefore heating it constitutes a significant change and is Melachah on Shabbos.
Ilan Segal