Water on back seat floor

Take out the door jamb floor pieces and pull back the carpeting and check for moisture around the center post behind the drivers seat. Then take it to a car wash and run it through and check again. This is most likely a door seal that is allowing water to enter the vehicle and run down to the floor and gather at the center post.

If passenger side is dry it would point to a problem with the door seals on the drivers side either a door or sunroof if you have one. If there is that much water you could sit in the car, have someone use a garden hose or pressure washer on the door and sunroof to see where the leak comes from.

Have you lifted the carpet up in those areas to check for a rubber plug?

On a dry day take a garden hose and shoot water in any area that could leak and check and see if there is water in any of those areas. Also check under the car to see if there are holes or cracks that could leak. A rust spot will sometimes leak as well.

could be your heater core check your coolant levels.... or you can have some holes in your floors

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.

Most likely the sunroof drains are clogged. It happened to mine, the floormats and floors were soaked, especially behind the drivers side. Volvo wanted to charge me $550 to clear them out and dry out the floors. Instead I took it home, rolled down the windows, parked it in the sun and let it dry out. Then I found a mechanic who blew out the drains with compressed air for only $20. He had originally quoted me $50-100, but ended up doing it for less. He said the two back drains are clogged. You can try sticking a small flexible wire down the front two yourself, then pour water down to see if it drains properly out the bottom of the car. When I looked I didn't see the two back sunroof drains, but my mechanic said they were clogged with leaves, you might be able to clear these yourself too. I would have never guessed that it was the sunroof causing this, because you don't see the water leak around the sunroof, you only see the water backing up on the floor. Don't take it to the dealership, it's a simple job and they will rip you off.

Remove the Cowl vent panel under the wipers. Each corner in the basin has a square drainage hole. Make sure they are clear with a blower or long wire. They get clogged with leaves and debris. Test it by pouring water and see if it drains under the fender. Drill additional drain holes if you want, at the bottom of this metal basin. On the back of this basin you will see a large rectangular hole where the air is sucked in for the heater. That is where the water gets into if the drainage is clogged. It then shoots through the vent under the drivers seat, aiming to the rear. that is why you get a wet rear floor.

more than likely to be either front or rear windscreen, water seeps under carpets to lowest part of floor, if vehicle is a hatch back check the channel at top of door opening is not blocked with leaves or debris as this can cause water to overflow and into vehicle

take the cover off of the door behind the outside mirror. if there is a hole where the cable goes through.. seal it was some type of elastic caulk such as silicone. water comes in around this when it rains but generally just runs down the door post and onto the carpet