Sounds to me like you smoked your radio and depthfinder.
Alternator regulators increase alternator output as battery voltage decreases. No battery = zero voltage = maximum alternator output. Apparently way more output than your radio or depthfinder could tolerate
Probably not what you wanted to hear, but it sounds like you need a new radio and a new depthfinder. You'll want to have the outboard's alternator checked, as well, because you might have damaged one-or-more of its diodes.
If the outboard's alternator has an alternator field input that can be disconnected, look into a new battery switch--one that has alternator field disconnect (AFD). That way if the battery is off, no voltage from the alternator.