Why is there water leaking on the passenger side floorboard of your ford focus?
(Fixed with regular old packaging tape)
First off, the leak is due to water getting inside the air conditioner's air intake. Water runs down the passenger side windshield and flows between the windshield and the crowl (the crowl is the big flat plastic seal connected to the bottom of the windshield). Therefore, Ford put in a gutter on the bottom-side of the crowl that is supposed to collect this water and move it elsewhere to a safe place.
The leak occurs because this gutter doesn't have a big enough lip on it to keep water from flowing over the gutter (similar to how a bent-down rooftop gutter would overflow when it rains). This water flows into the air conditioner's air intake, which is located directly beneath the crowl. Ford should have fixed this problem! They've had six generations or more cars with this same problem.
Instead of fixing this gutter (by adding a bigger lip on it), I decided to start with the easier route of covering half of the air intake with packaging tape. To do this, you need to lift up the hood of the car and lift up the crowl. There are 3 clips on the crowl that need to be unclipped and you can lift up the crowl afterwards (with a bit of wiggling) at least enough where you can get your arm underneath it.
Once you have the crowl up enough, you will see the air intake covered by a grating. I simply cleaned this surface and placed packaging tape over half of the grating (the half that is towards the center of the car). You need to make sure that the tape covers the grating and the grating's housing so that there are no gaps such that water can still flow into the air intake. I actually took out the entire housing so that it would be easier to clean and get to, but this is a little bit difficult and requires a wrench.
Also, I took a cheap plastic mechanical pencil (or some rod) and taped that down the center of the grating so that this would provide a lip so that water cannot get from the taped half of the grating to the untaped half of the grating. This might not be necessary, but easy enough to do.
We had heavy rains and I lifted up the crowl to see if my rigged-up fix actually worked and it worked much better than expected.
Most vehicle that have this problem, and being a ford man, it's your heater core! What's a heater core? In laymen terms, is't a miniture radiator that water come's from your main radiator cycles through the engine and in to your heater core and back through the radiator. You know when it's winter and it's cold and you turn on the heat inside the car, well that's how you get heat from the heater core. Either it's a loose hose connection ( doubt it) or you need to replace the heater core and that's not fun. That's where your problem is and you need to replace it now or flood the floor board of your car.
During the summer, it is NOT the heater core unless your car is a sauna. If it is only on the passenger side, I'll also bet it only happens when the air conditioner runs. Look under the car around the firewall on the passenger side for a small tube hanging down. This will be the drain for condensation on the AC. Carefully poke a wire up this hose to clear the blockage.
All good information but I had the identical problem, and it could be this. One morning after it had rained I had a puddle of water on the front passenger floor. I traced the water entry up to the openings where the heat would come out. First I thought it was the heater core or something but the water was clear, and it had rained. Then I thought it was the windshield as I had heard about some windshield seal problems. So the next Saturday I took the trim piece off which is just below the wiper on the passenger side to give it a look. When that was off I could see a cowl that is the air inlet for the heat and a/c and could look down and see the fan. So I next took a few cups of water and poured it around the cowl where the rainwater would normally flow and lo and behold, it started to drip into the fan area. The cowl was not sealed properly to the metal shroud that it was attached too. It was pretty easy to remove the two nuts and because the rubber gasket was still in good shape, I kept that and put a small bead of silicone on the gasket, cleaned the mating surface and put it back together. Problem solved for free in a couple of hours.