I may get a chance to repeat the measurement with the transducer in the neighbor’s swimming pool.
These fish detectors are designed to go in a boat with a 12 volt battery.  A 12 volt battery on a charger will measure up to 16.2 volts.  Under a light load and not being charged it will measure 14 to 15 volts (Ya I know a 12 volt car battery has 14 to 16 volts).   So these fish detectors are designed to work best on 14 to 16 volts although they will work somewhat down to about 8 volts. Alkaline batteries new measure 1.6V and drop to about 1.4 volts with little use and when 80% of the energy is used the voltage will be 1.15V. If 10 AA Alkaline batteries are connected in series the voltage will start at 16 volts and when 80% of the energy is used up the voltage will be 11.5V.  New Lithium AA batteries measure 1.82V and drop to 1.5V with little use and when 80% of the energy is used up the voltage will be about 1.38V.  If 9 AA Lithium batteries are connected in series the voltage will start at 16.4V and when 80% of the energy is used up the voltage will be about 12.4V.