Is that near midships on the side? If so, I vote rubrail. I had to redo mine a couple years ago. Had water coming in after an offshore trip. Seems that the spray or waves and forward momentum put enough pressure to force it in this area.
Just removed the screws, cleaned, recaulked and no more issue.
My rubrail is all rubber, so best to do in warm to hot weather since cold weather might make it hard to work with, it not impossible.