There are lots of places that rainwater can enter and end up on the passenger side front floor. One of the most common is a windshield that is not fully sealed along the edge. Also the door gaskets can cause this.
It may help to look for traces of the intrusion, tracks or trails of where the water came from. Get your head under the dash and look aroud with a strong flashlight. If necessary, have someone on the outside run a hose over the general area while you are inside with a flashlight looking at some of the suspect areas.
In extreme cases, remove the seat and carpet, dry everything out, then sprinkle talcum powder around the floor, set up a sprinkler to shower the car in the general area in question for about 15 minutes, then check the powder for an indication of the water trail.
