I had the same troubles with left side footwell. Clean the cowling drains if there’s debris from leaves and whatever. That didn’t solve mine since it was leaking through the door. The water deflector above the door speaker wasn’t doing the job. Water would pool at the bottom of the speaker enclosure and drain into the car instead of the drain hole at the door bottom. Took the door cover off and draped plastic behind the speaker enclosure to deflect the water. Dry floor ever since.
Link that solved this for me:
https://www.rx8club.com/rx-8-discuss...-fixed-167285/