I have a water leak on the passanger side floor. It occurs only when it rains.

Look around the windscreen area and look up from the floor mat area (as best you can) to determine if water is leaking in around the windscreen seal. Is there any rust around the windscreen area or body work? If you have a sun roof there is a drain tube at each corner of the roof to channel away water. A blocked drain may cause water to fall from the sunroof area Get someone to spray water around the windscreen and sunroof area with a garden hose while you look under the dash area with a flash light.