My driver's side carpet was awfully wet. After some prodding and poking I managed to find the emergency drainage hole.
They weren't mentioned in the Haynes manual and I could find no reference to them online. So in case you are wondering, there is one on each side, you have to cut open the carpet to find them. It's just a round metal cover plate (about 2' in diameter) sitting on top of a hole straight through the floor panel (about 1 1/2″ in diameter), about 9″ in front of each seat and 3″ in from the door.
The cover plate/plug is normally glued into place. I assume that if the interior fills with water and you want to empty it in a hurry, you just pop it out from underneath
My car must have hit something, because the rim of the hole was partly bent downwards (go figure) and the cover plate/plug was bent almost in two, thus leaving a nice big unsealed gap to really scoop up the puddles.
I hammered the cover plate flat again, used the car jack to press the rim of the hole back up flush with the rest of the floor panel and then gummed it all up with silicone sealant. I'll glue the carpet back down once the sealant has dried properly and pop the nice new mat back on top. Fingers crossed!