During a heavy or prolonged rain (or carwash) water collects on the rear interior floor boards. There appear to be two major causes for this - mine had the more frequent of the two. One is that the sunroof drains can get clogged. The drains run down the c-pillars just behind the rear doors and if clogged the water backs up and runs down onto the floors. The other, more frequent issue is a torn or unglued noise baffle/vapor barrier that is in each rear door. Rain water goes down inside the door at the bottom of the glass and that vapor barrier directs the water flow to a drain at the bottom-front of the door. The glue/sealant that holds that barrier to the door frame can go bad - when that happens the water goes inside the car rather than down the drain. Re-seal the barrier and the problem goes away. Be careful to do research, or take it to a pro if you have rear door airbags - if they deploy while your head is in front of the door it can kill you. For a car w/o airbags like mine - it took me about 20 minutes for each rear door to remove the door panel and put new sealant on the barrier.
I live in socal so we don't get much rain but when it does i know to keep the car in the garage which is very annoying. Otherwise, the floor boards get drenched with water build up from the cheap rubber seals along the rear doors go bad quickly.