Hallå Patrik, I have not heard of leaks along the rub-rail being a problem on these boats before. Is it possible it took a hard docking in this area ? Seems like that would usually occur further forward. Unless a dock neighbour made a mistake ! I would try to confirm this is the issue before taking it apart.
Can you use a high pressure water hose to track down the leak ?
I would have thought a leak at the rub rail would run down the inside of the liner and go into the bilge. While a leak around the port would come out on the bathroom vanity and end up on the floor. You could apply some talcum powder to the top of the vanity counter top while you are off the boat and look for some "tracks" next time you come aboard.
I believe there can be an issue with leaks around the lock downs on the ports. The fix is to take the little covers off the locks. Remove the screws and replace the O-rings, exact size is not that critical, just pick some up at the hardware store or SKF dealer.
If you do not have a dodger there could be a leak on the cabin top around the winches etc. that would then could come out through the light fixtures.
Good luck, and please keep us in the loop on the problem and how you fixed it, Bob