Hi Paul,
There is no definite answer to how you can fix this problem.
The first thing to do is locate where the water is actually coming in from.
The best thing to do is setup your garden hose, jump inside the car and get someone else to spray the side of the car that the footwell is wet. Ensure they pay particular attention to the seals around the doors and windows, also the rear light clusters and boot.
Use a torch to inspect the door seals, window seals etc for water coming into the car.
Inspect any rust patches (if any) on the car to make sure holes haven't been made.
Remember, just because water is in a particular footwell doesn't mean thats where the leak is. I recently had water in my rear passenger footwell, when I did this test we found the water to be coming from a rust patch under the rear light cluster, leaking down into the boot, under the passenger seat and into the footwell.
You may need to remove some of the internal covers and carpets to locate the leak. If this is the case unless your sure on how to do it yourself ask the help of someone who knows what they are doing. It's easy to snap the connectors or the covers when removing them if you don't know where the fixtures are.
Hope this helps, at least if you find the cause you can work on the answer.
Regards.
Dean
November 2006