Looking at the diagram and layout the seal on those rear windows would have to be the first item to check. I have however noticed that water runs down the deck of my Oceanis
and that area is a seemingly natural drain point so is it possible you have a leaky deck/hull seam or even the cleat/stanchion fittings?
Another consideration here (assuming your Oceanis is similar to my 50) is that there is a ledge at ceiling level in the rear cabins where cables
run from your instruments in the cockpit
to the nav table so there is a possibility that water is getting in around a stanchion/cleat fitting further back or where the rear winches or lockers are and running along this ledge.
Just a few thoughts, good luck