On my 95 M3; What would be the cause of there being water in the passenger side floor board? It happens only when I wash the car. Where could it be coming from? Is something leaking or clogged? Need help. Thanks

I can suggest a few places that might make sense assuming you don't have a seal issue around the window glass or door frame. The vent area will fill with water if it is plugged with leaves or the like. Be careful as you could be getting water into your ECU area too and that will fry it. The ECU area is right in front of the passenger side and against the firewall. Look for a large 12 x 14 rubber area along the firewall and be sure you have that sealed too. Many 3 series have been fried by the water getting into the ECU area.
Might be related, might not.
I provided the most common cause as I am willing to bet the heater/AC unit drain is not plugged right?