Can you expand on that?
Genuine question, I have no idea about the technicality of the subject.
Water is unable to be compressed, ask anyone who has hydrauliced an engine, so it makes a great hydraulic fluid. However it has a very low boiling point. At boiling point it transitions from liquid to gas, in gas form, it has a higher volume, thus causing expansion in the closed system, and either locking the brakes on at the worst possible moment, or rupturing the circuit at its weakest point, causing loss of breaking. NB: brakes will most likely lock on a long descent, and the front will likely lock first, killing or maiming you.
Conversely, water has a very high freezing point, the point where it transitions from liquid to solid. Ice has a higher volume than water, causing the same concerns as above, the only mitigating part is, the brakes are likely to lock or rupture when the bike is stationary. However if the system doesn't fail at the time of freezing, it will be, at the very least, compromised, weakening the circuit, exacerbating the likelihood of failure close to boiling point.
DOT fluid is glycol based, which if you've ever read a label on a bottle of coolant (anti freeze/anti boil) is what stop your engine from overheating or freezing solid.
Mineral oil also has a boiling point similar to glycol fluids, around the 360°c point and a freezing post of about -30°c.
Water actuated brakes are the worst idea since press fit BBs.
#knuckleswasadrunkenchunt