I was told that if you let the transducer operate in air there is no place for the energy to go so it damages the transducer. I do not think shooting through the hull hurts as long as the space between the transducer and the hull is filled with a material that will transmit the energy well and the hull is made out of a material that will transmit the energy (not foam core).
I think it was KF that recommended the X-4 although I like it also.