I had a water leak in the car. Boot was not soaking but deffo water in somewhere. Rear passenger side floor drenched, just full of water. No other water sitting in the car.

I checked the lights, the foam seals round the lights, behind the plate, round the boot and door trim and seals, boot and main floor, sunroof etc and still nothing.

Where was it getting in?

Well, the rear of the car was slightly hit when I got it and I finally noticed a minute crack in the seam sealer below one of the rear lights - the bit vertical and joins the wrap around bit from the back wing to the rectangle bit below the light and lo and behold there was my problem. A squirt of automotive polyurothane sealant and it was sorted. I put a good bit inside and rubbed a bit into the outside of the crack.
Once it had set a bit, a rag and meths took all excess of the outside paintwork and there's only a hairline of black visible and you really have to be looking to see it.
No more water leak!

Just shows you!