Hi Cal,

Electrical noise issues are more frequently grounding deficiencies rather than the cabling.  Your depth sounder transducer cable should be shielded to prevent noise impression on the signal, and the shield connected to depth sounder negative of it's power supply.  If it is not than a better depth sounder would be in order for your situation.  While it can be done, it is often difficult to add shielding that is contiguous without RF gaps.

Good grounding practice is the most effective mitigation.  Single point grounding, where all the common points (ie: negative power and signal wiring) go to one point is essential:  motor controller & depth sounder controller especially connected to ONE SINGLE point only at or near the battery common.

The other practice that will reduce noise impression is keeping power wiring away from signal cabling, and have signal cables cross power cables at 90 deg.

Finally adding a noise filter in the depth sounder power cable will reduce anything coming from the power source.  These are readily available in a small package for low current devices in a case with a ground terminal connected to the single point grounding point. 
https://www.jameco.com/z/851-10-007-Qualtek-AC-Power-Line-EMI-Filter-115-250-VAC-10-Amp-RMS-Max-50-60-Hz-Flange-Mount_2230176.html

Hope this helps you.
John A