A lot of talk here of audio signal generators.

Although depth sounders are referred to as "sonar" they do not work at audio
frequencies.

Hi-Fi equipment can work up to 20K (salesmen and status-seekers will clain
more)
This is the upper level for people with good hearing. By the time we can
afford boats and gadgets out hearing has deteriorated to much below this.

Modern depth sounders work at 200K. This is radio frequency.

Some years ago the most popular channel on BBC radio was on 200k. It would
seem that, if you wish to test your fishfinder, you might be better off
using a thirty year old European broadcast receiver and hold the antenna to
the transducer.

Henry