David passes along that you may be getting noise from the alternator when the engine is on. Could be any number of things along the way (to the depth sounder) but corrosion in connectors, terminals, etc. Go through and check for corrosion and clean up everything.
You can get out your computer with a serial port--your NMEA 0183 signal will be able to be dumped to the computer (it's text) and you'll be able to tell if it's giving you something like um...depths. If all scrambled, then noise in the signal is likely.
Since it only happens when engine is on, the above is likely. If all the time:
If you have two depth transducers, you need to make sure they are running on different frequencies. Our FLS is different frequency than our DST. Can't recall the two different frequency bands, but you could look up each of your transducers and see if they both are running on the same frequency.
No, they don't tell you this stuff when you're buying the transducers either
PS--we like stand alone display rather than part of chart plotter. Our depth transducer runs to a CruzPro display inside that can be seen from cockpit. The FLS is in the cockpit and can be seen from there, of course. We note that the DSP is approx 30 feet behind the FLS. So, we often get two very different depths from the two different sensors.