The heater valve is most likely allowing water through even though it is in the closed position, they are not the best and do not seal billiantly, although some are better than others in my experience.
I would of course check that the valve is actually operating when you move the lever. Remove the battery and bulkhead insulation and you will see the valve. Get someone to work the lever and watch to make sure the valve is moving.
When moving you will always get air flow into the car whether the fan is on or not, shutting off the fresh air intake flap should help stop this though.