What speed does a dirt bike hydroplane?

The FAA did a lot of work to determine the factors involved in hydroplaning. Their testing showed that the speed at which hydroplaning can occur depends upon the air pressure in the tires.

The formula is

V = 9 * Sqrt(Pressure)

where V is speed in MPH and Pressure is in PSI

If you add weight but don't add air, the tire will compress, which causes tire pressure to go up, increasing the hydroplane speed.

ref http://www.sheldonbrown.com/tires.html#tread