Usually, as mentioned, there is a tube that contains your rudder shaft that runs from the bottomof the boat to the top and it is completely sealed off. If there is a crack in the tube, or if the top end of the tube is not sealed so rain water or wash water gets around it, that could be the problem. Most likely you have wheel steering in which case the tube (usually fiberglass) is cut partway up and a section removed to accommodate a radial or quadrant which are pieces of the steering system below decks. If this is the case, then you have a rather large stuffing box (like the one on your engine shaft) that holds out the sea/lake/river water from below. Like a shaft stuffing box, it has a packing gland which sometimes needs replacement, or at the very least the nut on the stuffing box needs tightening, which compresses the gland more to eliminate leaking. These are things I would look for first.
