Not being a mechanic I am trying to work out the reasoning of this problem. So, thinking out loud:
Logically, the speed is going to be governed by the on-board computer receiving information from the driven wheels. The computer will be calibrated to calculate the speed from the given circumference of the tyre. (This is also a reason why a worn or under-inflated tyre can give a false speed reading).
So, when you aqua-plane the tyre loses contact with the road. But, the on-board computer will (should) still keep the wheels turning at the correct speed.
Therefore, why should the car gain speed? If it is in fact aqua-planing there the driven wheels do not have contact with the road to increase the cars speed. Perhaps the absence of friction between the car tyre/road can momentarily increase the forward momentum?
On another point, ever wondered how the "flat tyre" warning light operates? The on-board computer measure the rotation of all tyres - if one tyre rotates faster than the other(s) then this indicates a loss of tyre pressure, and hence the warning light comes on.
