A: There are several causes for this. One of the most common is water draining from the A/C onto the passenger side floor. It is most common when the A/C is on and you have been driving at a high speed for a while when it is real humid outside. There is a drain hose in the firewall that sticks straight out and the air keeps the water from draining out of the hose, so instead it drains inside. There is a TSB from Jeep on this, so it is a known problem. If your still under warranty, go to the dealer and they will fix it for free. If your out of warranty, you can either go to the dealer and buy the hose ($5-10) and install it yourself or just find any 3/4" hose and stick it on like in THIS PICTURE.

There is also another common cause for this, the drain hose may be clogged up with mud/debris. Just take a blunt stick (a unsharpened pencil or a chopstick work very well) and stick it into the tube and make sure it is clean. You should do this before you put the elbow on to make sure it is not clogged up.

These are not the only causes that can create the water on the floor. The windshield seal can be leaking (usualy only happens if you have folded down the windshield), the seal around the door could be leaking, or the cowl drain (under the vent hole just behind the hood, can be fixed by taking a waterhose and try to flush out any gunk in there. If it is real bad, you will need to remove the cowl and clean it out with your hands).