check the seals in the door, window, and windshield on that side. put paper towels (or something to catch water) next time it rains, see if you can find a pattern of entry.
One reason your floorboard is wet after it rains is that there may be rust holes in the floor or the floor pans are rusted out. A leak in the windshield seal may also be the cause of wet floorboards. Bad door seals or window seals can allow water to soak the floorboards when it rains.
water in passenger floorboard, where is the hose that would cause this?
Either your windshield or driver's side window has a major leak
A damaged heater core will cause coolant or water to leak onto the passenger floorboard.
Water.
If you left the vent open, or if the vent under the windshield is full of leaves, it could cause rainwater to flow into the floorboard.
it rains
Since it happens only when it rains, it would suggest a few possibilities. Look for these:Bad window or door seals.Bad windshield seal.Holes in the floorboard or firewall, especially near the wheelwells.
Check AC drain and clear the blockage. Also look for firewall sealer skips.
I had the same thing happen on my '87. It started after the windshield was replaced & something wasn't done right there that let water in at the bottom of it.
the ac condensation drain hose is plugged or crimped.
open sun roof