Rep Power: 0

Passenger side floor is filling up with water when air conditioner runs. What do I do?

 
Rep Power: 308

Sounds like your drain tube is plugged in some way.

Not sure of the best way to clear. Maybe ezone will chime in.

It is near the firewall area underneath, a black tube hanging down.

 
Rep Power: 518

Drain hose/tube either clogged or disconnected. I have no idea what your car has without looking. Stick your head under the dash and take a look while it's dripping. If the drain tube is clogged, remove it from the bottom of the evap case and either poke it out or use an air gun to blow through it... If the clog is inside the plastic housing, that's a lot more work... I used to see them when people kicked the hose loose from the bottom of the evaporator case. Simply reinstall, maybe wrap a hose clamp around it.

 
Rep Power: 308

So the evap drain and the top of the drain hose is easy to access in the footwell area ??

 
Rep Power: 518

I have no idea. Some are, some aren't. I can't see the car, I don't know what car OP has, and there aren't any here on the premises handy to look at.