Hi there, thanks for the question.
Generally when you see large amounts of water in the front floor boards it's coming from either the front drains from the sunroof, or from the cowl drains, or windshield seal itself.
For the sunroof you need just pour water in the front track and watch for it to dump behind the front fenders underneath the car. If you don't see any water draining then the drains are clogged and causing the water to drain in the front panel area.
For the windshield and cowl you want to run water across the windshiled and watch for leaking at the front floor boards. If you have water intrusion you need to pull the wiper cowl and check the drains as well as the bottom windshield seal.