My husband has water on the rear floor board on the passenger side of the car when it rains. This happens if we drive or park the car during a heavy rain.
Could be the back glass leaking and running down into the floorboard. (more accurately, the seal on the back glass)
Do you have a sun roof. That is the most likely problem. They tend to leak when the drains get clogged and you can have water come on on either side front or back.
The careful use of compressed air or likewise careful use of a length of string trimmer line from the garden center, will usually take care of it.
I have a 2000 Blazer that had a leak in the seam between the firewall and the inner fender. The water was running down the seam underneath the carpet and padding and coming up through the carpet in the rear floor board. I found the leak by shining a flashlight underneath the dash while running water into the front cowl. The light reflects off any water getting inside the vehicle.