It could be the seals around the door, check for any breaks in it. Another common problem is the membrane behind the door card, whereby the 'mastic' which holds the membrane to the door has come unstuck allowing water from the window sill to leak into the passenger compartment.
Those are the two most common problems causing leaking into the footwells.
Hope this helps. If you cannot see there is any damage to either of the above do a search in the forum, there are plenty of posts about leaks that may help you.