I think we can safely rule out the AC drain here guys... the water leak happens when its raining and when the car is being washed according to the OP, that's kind of a clear indication to say that this might not be from the AC.
Apart from the causes the others have mentioned, let me point you to another possible cause that might cause water to get into the car. When a considerable amount of water is sliding down your windscreen (when it rains, and when the car is being washed), it goes through the grill on the windscreen garnish (shown below), and the water is directed towards the fenders through a sort of "gutter", and then to the ground through a small passage.
Pic 1: The garnish below the winscreen
Pic 2: Garnish removed, "gutter" exposed.
Pic 3: Close-up of the passage (the arms you see here are for the wiper mechanism)
With time, dirt, tree leaves and the sort get trapped inside this place causing it to corrode slowly. A tiny hole in this passage will result in water directly flowing into your front floor board (depending on which side the hole is). The other factor is that the fresh air vent is also located on the passenger side firewall and faces this passage direcly. There is a chance of water getting into the system from there as well.
You can inspect this at home...
1. Take off the wipers
2. Take off the garnish below the windscreen (it's fastened onto the body with plastic clips)
3. Inspect the passage for leaks.
This is just one possible cause. Hope this helps. Good luck!