For what it's worth: A depth sounder will not work if the boat is OUT of the water. The transducer is designed to send signals through water..not air.
Also, when the boat has been out of the water for any length of time..say all winter, the down facing surface of the transducer may have to be "wetted" for it to work imediately. Smear a dab of dish washing liquid on face of the transducer just before launching.
NB