Since I am chasing (so far unsuccessfully) my own 12V gremlin I won't offer any suggestions on what may be causing the problem. However, until you can sort it out you might want to consider a battery solely for the depth finder and recharge as necessary at home.
_________________________
Remember to immediately record your catch or you may become the catch!

It's the person who has done nothing who is sure nothing can be done. (Ewing)