Other days trolling with the outboard is definitely the ticket. I am not so sure underwater exhaust is a bad thing though. The sound is dispersed through the water along with all the other sounds. Remember that sound travels much better through water, since it is a denser medium, than through air. Down rigger cables vibrate creating sound as do lures, dodgers, fishing line, etc.
Fish respond favorably to some sounds.
One advantage you have with through hull exhaust is less noise and exhaust in the boat. Those are a couple of real advantages. This has several advantages. Fewer fish are broken off if not played well. Less tackle is lost.