First off, you should never leave your accessories on when you start your big motor. Power surges like that are death to modern electronics. When your voltage drops to the accessories (like Depth Finder, radio, GPS, etc), your amperage increases ... that plays hell with those things. That's my guess anyway (unless you have some sort of isolator and two batteries). I assume your feed wires are a heavy enough guage for the task (?). I'd guess minimum 12 guage.

My suggestion is to have a master switch for all of your accessories, flip it off when starting, flip it back on when running (IF you're only running one battery).