Most likely it would be the seal on the drivers side door. If the seal is broken or worn near the top edge, the water can trickle down to the bottom of the door without you noticing and actually well up on the inner side of the door sill, when that overflows it trickles into the footwell. When it rains, if alot of water falls to the ground when you open the door, this is your problem. Door seal is quite cheap and easy to replace.