It will happen once it starts doing so, no matter what you do...
basically its not your seals or rubber, its your car's body, some where, its rust that creates a hole that allows water to enter, it could be something on your pillar that eventually drains on your floor, it could be in the engine bay, check aound your wind screen, the roof mouldings, the point where ball joints meet, anywhere at all, once their is a small whole, it creats path for water to enter, travel across and eventually drain on the floor,
If you really want to catch, just clean the car from the inside, if you can, use a pressure washer, point to point target the edges of the car, point at each point for about 3 minutes to enable water to travel down, you may be able to find the point, usually its around the wind screen, roof mouldings or the upper side of diggi that drains water onto the left floor bed. next move to the underside front edge behind the axle...
where ever the entry point is, get it welded and coated.