Could be any of those things or the door seals. Perhaps you have hole rotted through somewhere.

One thought, though. Are you sure it's water, or could it possibly be coolant? Are you losing coolant? The heater pipes and such are up under your dash and if they're leaking, your floor-boards would be wet. I'm just guessing here, but it's possible, I suppose, that you're only experiencing this during rainy days because you're only turning up the heat during those days.

Just a couple thoughts for you. I hope it's just a firewall seal or something.

?_________
|_|_|\____\___
l-----[O]≡≡≡≡[O]
()_)()_)-----)_)

Stay the Trail!