Location, location...
Hey James, I installed an in-hull transponder on a H25 I owned three years ago and it still hasn't missed a beat.  I located in the starboard storage compartment under the V berth as far aft as possible.  It was about a foot below the water line and you have to factor that into the reading in the shallows.  I located the readout on the exterior starboard cabin bulkhead over the compass.  I used a Uniden 100 which sells for about $100 at BoatUS or West Marine.  No holes in the hull,,,  No problems.  Good LuckAl Britos/v SHOWTIME