As usual I will offer a different possibility/angle...
Many OBs and most stern drives are installed with a overiding protection against starting in gear. Normally a microswitch in the gear/throttle assembly wired to the starter solenoid.
Something to check. I can't remember if it's a wired as continuity or the other way round though......let alone whether your engine has it!
Obviously if you are getting a large voltage drop but no action when you turn the key it's back to the main wiring or SM but if you get no voltage drop this is where my money would go....
FORGET THIS - just reread your post re the fishfinder etc - you are clearly wired for current flow so it's back to connections, state of the wiring, or, if you have one, the battery isolator switch - if you have one then this is now where my money rides!