To reduce hydroplaning risks
Never use cruise control in wet weather driving conditions.
Why should you turn off your cruise control?
When the road is slippery, turn off your cruise control system. Snow, ice, slush or even rain can cause wheel-spin and loss of control.
The only way to stop this wheel-spin and maintain control is to immediately reduce power. However, an activated cruise control system will continue to apply power, keeping the wheels spinning. By the time you disengage the cruise control, you may have lost control.
For more information:
http://www.wikihow.com/Drive-Safely-in-the-Rain
http://www.nws.noaa.gov/os/severeweather/index.shtml