Hydroplaning creates more drag on the wheels, which is evenly balanced by less effective brake drag, thus creating a similar deceleration force compared to a dry runway.

The lower effective braking force has a much greater effect on increasing the landing distance than the benefit of higher drag. 

The effect of the increased wheel drag through the water is far greater than the loss of friction and reduced braking effect, therefore landing distances decrease. 

The coefficient of friction is dramatically increased, which reduces the maximum braking energy that can be applied, which increases the landing distance.