Under the vents at the outside base of the windshield, there's a well. This well drains via a rubber tube that comes out of the firewall into the engine compartment. This tube is flat with a wide slit-like opening, and it therefore clogs easily with the debris that falls down into the well. When this happens, the water can't drain and eventually makes its way to your floorboard. When you open the hood, the rubber tube is on the firewall on the passenger side, right behind the engine block. It pulls off and goes back on pretty easily. Just ram a coat hanger or something through it to clean it out. I bet this will fix your problem.