First thing you need to do is figure out WHERE the water is getting in. If it's coming in through the prop shaft, you could fill the shaft tube with water proof grease . If it's coming through the rudder post, you can do the same thing though, to be honest, I doubt that would be the issue. Other locations you could be taking on water are where the anchor goes through the hull, the rear deck hatch or under the cabin.
I would start by putting the boat in the bathtub, with the cabin and rear deck hatch off and watch to see if any water is coming in from either the rudder post or the prop shaft.