Check the lower gudgeon
I had a similar problem. Turns out that sometimes when sailing, and often when motoring, the lower gudgeon is underwater and leaks. The gudgeons are the things that the rudder connects to on the transom. I purchased some rubber gasket material and made a gasket the same size as the gudgeon with holes poked through (using an ice pick) for the bolts. It works great, not a drop of water at all this season. I found the leak by placing dry towels in a few spots and seeing which one got wet. It was the towel aft of the keel area and that led me to the gudgeon. Don't be fooled by the fact that the lazarette where the bolts come through may be dry. The water travels down the interior of the transom directly to the bottom of the hull and then runs down to the keel/bilge area. Best of luck. Randy