On one of the Top Gear specials, they shot a hole through the floor. That soon drained the water out.

Seriously though, you need to find where it is coming in. On the passenger side it could be door seals, door membrane, sunroof drain pip come adrift, rust around the base of the windscreen causing the seal not to, err seal or though the bulkhead. Check to see if the scuttle drains are clear and water is not collecting by the wiper motor for the last one.