What would cause water to be on a Chevy Impala back floor?

You have had the door not closed all the way or the window cracked open just a little.I would also inspect the door for proper adjustment and seal fit. Take a dollar bill and close the door seal on the dollar bill and see how much drag the dollar bill needs to pull it out. Try it in all different places on the door. You can also mix up a squirt bottle of water with a good amount of soap in it. Then close all the doors and windows and turn the blower fan on the highest setting with the recirculation button. THen spray the soapy water all around the door window seals and see if it blows bubbles anywhere. Let me know what you find.