Update on leakage situation ....... First of all thanks for the suggestions, I'm no mechanic or bodyshop person, so I took the car to my local Nissan dealership to have them unclog whatever was clogged.
They tell me that they don't see water leaking into the car (after soaking it with a hose for 20m). They can obviously see the soaked floor of the driver's side, both front and rear passenger. The service guys tells me that if they can't see the water coming into the vehicle, then they can't diagnose what's clogged and therefore they can't fix it.
He suggested the next time I have water in the car to stop by so that they can assess the situation. I don't see how that would be any better. You never see the water leaking into the car, you just see the accumulation.
There is water in the trunk so I guess the suggestion about the hoses that are there are probably correct. Thanks,again. At least now I have an idea of where to look for the solution.
Now I'm off to pick up my wet car. Let's see if they try to sell me a new vehicle instead of fixing the problem. I've got nearly 250,000km on it and it still drives like new (so as long as the floor doesn't rot out, I'll be driving it for a couple more years).