Been There, Done That. Art1966 And Manolito, I've Had A Leak As Described And Had No Plugged Drains, But Did Find A Void In The Butyl Sealer At The Bottom Of The "Water Deflector".
It probably makes a difference as to the make and model of the vehicle, but I can assure you it's possible to have a leak caused by a loose water deflector and not have plugged drains in the door bottom.
Another thing that can make a difference is the angle at which the vehicle is parked. Some leaks of this type are triggered by parking on an incline and it can even matter whether the car is backed in or not.
My fairly severe leak was completely remedied by doing nothing more than finding evidence of the void in the butyl sealer and fixing it as described in a previous comment. The hard part is finding the water leak source and mopping up. The repair is rather easy.