Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
First, trace the whole system through, the lines that is, from the Garmin to the sonar transponder.
For each connection, undo it, then brush each side of the connector with a clean dry brush, for 20 + seconds each, then wipe down each side of the connector with a clean dry rag cloth until the metal shines, then re-connect firmly, then go on to the next connection and do the same brush-and-wipe for all connectors and connections along the entire system (with the power Off, of course.)
This is a treatment for corrosion. Which happens often and threatens the proper operation of all electronic equipment and connections in a maritime or lakegoing or riparian environment.
Let me know how it goes, please. Thanks... I hope you will see improved performance... and I will have more suggestions, if this does not work for you. If it does work, let me know.