Hey,
It's a small world.
I have an electric only boat also.
Do you have a Ray 48 volt?
You say your deph finder is separate from, but how is it separate?
If you've got your sonar hooked to one of the four 12-volt batteries
that power your big electric, then that's a big no no.
Also, your sonar wires may be too close to the wires for your big electric.
I know you don't need any additional batteries, but get a small marine
battery just for your sonar.
Try and run the sonar wires away from your motor wires and
you will be good to go.