Water under driver side floor

A simple test is to feel around the carpet in that footwell. See how far up under the dash the carpet is wet. Does it only happen after rain? Do you have a sun roof fitted?
If the carpet is only wet low down on the floor the most likely reason is either a perished or damaged door seal rubber, a faulty sun roof seal or the drain channels for the sun roof are clogged up. Check the footwell in the back as water always flows and settles at the lowest point in the vehicle.
If it is high up and nearest the middle of the dash you issue could be the heater matrix under the dash but as it seems to be only on one side this might not be the issue.
Door seals are a simple case of pull the old one off and push the new one on and they don't cost silly money to buy. Heater matrix issues, unfortunately, usually involve taking the whole dashboard out to get to it.
Hope this helps.

check drain holes in the bottom of the drivers side door. If they are clogged the water will build up and drain from the sides and on the floor. Also check the gaskets around the bottom of the door.. You may have to get on the ground and look up to check them... something that people seldom pay attention to.

Sounds like the windshield is leaking. Run water over the windshield and have someone get inside car and look for the leak. Alternatively take it to a reputable windshield repair shop and have them check it.

how old vechicle could have rusted out floor panel allowing water come through floor panel when raining driving on wet roads.

a couple things to look at if it has a sunroof the sun roof would have drain hoses that run down in each corner next you need to look at the cowl this is the section in front of the windshield that has a drain on it as well if you find nothing there the next step would be to remove the seats and carpet and sit in the car while running water over it either way you should remove the carpet and make sure it is dry all the way threw or mold will start to grow and you will never get ride of it not good for your heath other key spots to look at is if the windshield was ever replaced it could be a poor seal

Do you have a sun roof ? if yes make sure water is not leaking from there, BUT if you don't have a sunroof then have a look under the car's body, there might be some rusty area of a hole under the body letting the water from the road to get in.

have the rubber seal around your windshield check by a wind shild installer base on what you explain the sel at that corner is leaking and the water is leaking back into the vehicle

Hi, we also have a 2002 Toyota Rav4 and developed a leak in the front driver's side of the car. After much research, we discovered that the drain wells from our sun/moon roof were clogged. If you have one of these, I suggest that you open the sun roof and look carefully. You will notice some tiny little holes in front and in the rear of the sun/moon roof tracks. Over time, these become clogged with dust and debris. You need to find a long, sturdy piece of thin wire and run it down the holes. To test if you have properly unclogged these, take a small amount of water and pour it down the holes. You should see water coming out of the drain wells under the car around the rear of the front tires. Hope this helps.

I am the original owner of camry xle 1998 with wet floors. In 2000 I had drivers side door seal leaking at welt. Removed old door seal and new weather strip on drivers side installed. The door panel was never wet, but wet at bottom of door where there are drain holes...slowly leaking under carpet.
In 2001 later water again. Coming from overflowing A/C evaporator-drain plugged/ tank full of debris. Removed evaporator, cleaned + drain cleaned.
In 2003, driver door seal against window rotted. Removed door panel and found leak at window belt molding at the mirror joint inside + out. Lots of mold under carpet needed to replace! Made a fuss at dealers for reacurring problem and they replaced the carpet for free.