Merv, I was thinking that a bit of rainwater will be dried out by the exhaust heat rising through the floor from underneath. The trick would be to make sure you don't park the car in the shed for weeks after a getting caught in a deluge - which only happens in Melbourne about once a year!
You can also buy marine "midget" vents that you could mount in the floor to allow for drainage if you're really concerned. See below. They might even help draw air through the footwells and keep the temps down.
Cheers,
Paul