I have a 99 impreza and when it rains water leaky onto the passenger side floor board under the dashboard. I had a 96 impreza that also had the same problem. Has anyone experienced this similar problem and found a remedy?

It very well could be a leak around the pinchweld of the windshield.

There is a drain hole in the cowling just under the hood in the corner in front of the windshield on both the driver and passenger side. It gets clogged with leaves and other crap. Make certain the hole is patent and try to pour some water in this area and see if it drains out under the car. If not, the rain water that should be draining out is overflowing and going into the passenger compartment probably through an air intake.

I have the same affect when I use my A/C. Mine is from a broken drain tube. I really have been meaning to fix that because shop-vac'ing the carpet on a daily basis is getting old quick. Maybe it will get cold here soon and I can be lazy. It's FL so I doubt it.