Matthew, I do not know if your hull design is similar to the 400DB, but I had a similar problem. The leak turned out to be behind the upper rubrail end where it ends and buts into the transom. I had two problems. First, there is an epoxy "weld" at the vertical hull seam behind the cosmetic caulking. That epoxy did not exist behind the rubrail. It was as if they installed the rubrail first then epoxied the seam second. So there was a gap there. Second, there were extra empty screw holes where they must have trial and error drove long screws in to pull the end of the white rub rail tight to the hull around the curve. Also, the cut end of the rubrail was caulked sealing water from running out when it go into the rubrail channel (and water does get in there no matter how much you caulk the whole length of the rubrail). With this being the low point of the full length of the rubrail, there was plenty of opportunity for water to dam up at the end and leak in through the gaps and extra holes.
