Your cruise control uses data from the Vehicle Speed Sensor to decide how fast you're going. The VSS is usually hooked up to the speedometer which is usually driven from the output shaft of the transmission. To keep it simple say it's a RWD car-- the speedometer really measures the speed of the driveshaft before the power is sent to the differential and distributed to the wheels. Now, it is possible in deep water or other slippery conditions that if one tire loses traction but the other one doesn't, that the speed being measured by the VSS will go down and the cruise control will respond by giving the engine more throttle, which will lead to a spectacular spin, and possibly a fiery crash.
The above scenario isn't too likely, however, because it's far more likely that if a wheel loses traction, the driveshaft will speed up and the cruise control will let off on the throttle. So you're generally okay on "normal" wet roads like Craig says, but it's probably not a great idea to be using cruise control on really bad roads.