The cruise control is designed for normal road conditions. It doesn't know when the pavement is slippery. Under slick conditions, you need to be in complete control and monitoring road conditions. You're more likely to notice hydroplaning if you are not relying on the cruise control.
With some vehicles, it is possible that the wheels will actually spin faster when the cruise control is on and the car hits a slippery spot. When the tires make contact with firm road again, the car can skid or lose control.
[I]On most vehichles, the cruise control is disengaged by tapping on the brake. In an emergency, this adds a fraction of a second to your response time as well as the risk of the braking action itself causing a loss of control on a slippery road.