After a long descent or hard driving, the brakes can get extremely hot, and it is tempting to cool them with water - especially at a car wash or after off-roading through a puddle. Doing it deliberately with cold water on very hot rotors is a bad idea. Here is why.
Thermal shock cracks rotors
Brake rotors can reach several hundred degrees under hard use - see how hot brakes get. Dumping cold water on that hot cast iron causes rapid, uneven contraction - thermal shock - which can crack the rotor or warp it into thickness variation that causes a shudder; see warped rotor causes. The bigger the temperature difference, the greater the risk.
Incidental water is usually fine
Driving through a puddle or normal rain hitting warm brakes is not the same as deliberately blasting glowing rotors with a hose - the water is spread out and the brakes are rarely at their peak temperature during normal driving. A little splash briefly steams off and the brakes dry in a few stops; see brakes squeak in the rain. It is the deliberate cold-quench of very hot rotors that does damage.
The right way to cool brakes
Let hot brakes cool gradually with airflow - keep rolling gently rather than parking with the brakes clamped hot, which can imprint pad deposits. On a long descent, use engine braking so they never get that hot in the first place; see why brakes overheat towing. Patience beats a cold hose.
If a rotor gets shocked
A rotor that has been thermally shocked may develop cracks or a pulsation - inspect it and replace if cracked or badly warped; see when to replace rotors. Fit new rotors as a pair with quality pads and fresh hardware.