Have you removed the plastics that go inside your wheel arches?
does it only happen after you have driven in the rain?
If you open the front door you will probably see your front wheel if you dont have the plastics there which will pick up water when you are driving and send it through that gap and then into your footwell....
Thats my theory anyways coz ive had the same problem.
I found this website which kinda goes through making some replacements and i'll be giving that a go hopefully soon.
If I have any luck making them i'll let you know