Very recently sorted this problem.
Water comes in through the scuttle area and down inside the bulkhead usually towards the drivers side, if you park on a slope - the water can creep into the rear floor area too.
The root cause was the centre scuttle drain getting blocked, filling with water and overflowing into the leaky a/c area, this drain 'pipe' which comes from the scuttle is difficult to see but you can get you hand down the back of the engine exactly in the centre just before where the deadening ends and the heatshield starts, you should find a small hole, poke you finger in and twiddle the rubbery flappy bit (like you do) - this becomes blocked, permanent cure is to tear the rubbery flap off.
I confimed this problem by removing the scuttle trim and filling it with water to see if it drains away - it didn't from the centre drain, until I attended to the flap.
The main problem is the existing water collected inside on the floor, this means taking the seats out among other things to remove the carpet and the spongy underlay which gets totally waterlogged.
With the sponge out of the car, it took me five sunny days to dry out the sponge completely and this is the only permanent fix, I am convinced that this sponge will NEVER dry out left under the carpet and may eventually cause corrosion.