First be sure your floor and firewall plugs are in. I get water on the floor when the plugs are out and I go through a puddle or just driving in heavy rain.
Lay a water hose on the hood next to the windshield and take a flashlight inside and look for it.
Areas that cause issues:
Cowl drain - center under cowl
Vent under cowl - see link
AC vent tube clogged.
Heater core leaking
Around heater core - where it goes though the firewall
Gasket between windshield and body/cowl
Top Windshield gasket between frame and top
http://www.jeepforum.com/forum/f9/stripped-plastic-cowl-nuts-caused-leak-696834/
Update - I ran a wire hanger down the drainage tube and did not appear to run into any significant blockage (little to no resistance as I fed the wire) ... I ran about half gallon of water down the grille and it all seemed to stream rapidly through the drain to the ground ....
I was at an auto parts store a couple of days later to buy a brake light switch ... was asking another Jeeper about the water intrusion into the footwell on the drivers side and he said that it might be the windshield ...
... the windshield ...??? I replied ...
He indicated the seal around the windshield can shrink up enough to allow leakage around the edges and it can run into the footwell area ....
Anyone else identify this as a particular issue or contributing factor in water leakage ...?
R/
'Chop