Re: 79 Evinrude 55 overcharging
if you want/need a flat 12v on your depth sounder (mine was a Humminbird "wide paramount" reacting to high voltage) you can buy a 12 volts regulator chip at Radiao Shack for a few $$ .
they are good to drive 1 amp flat output 12volts with inputs from 12-30 volts !!
If you need a higher current draw for your depth sounder model you can hook-up 2-3 regulators in parallel by soldering the 3 individual legs of the chips together (do not confuse the wires....wire each identical leg to the same chip legs to respect polarity).
Soldering these small legs together takes dexterity... Fat fingered guys can just wire extension wires on each chip leg ....then pigtail
the extension wires in parallel. This makes bolting the indiviual chips on the heat sink much easier

Cover/seal with stove pipe high heat resitant silicone
the chips have a mounting hole.. just line them up to mount on a 2 square inch aluminum plate for heat sinking
and wire in the ground , the battery +12v and... the last wire feeds the depth sounder for **** flat 12Volts.
Cover/seal with stove pipe high heat resitant silicone
Wire in a disconnect switch on the battery feed (or use INSTRUMENTS toggle switch feed) as these litlle chps will draw some milliamps even with the depth sounder off .
I never had HIGH VOLTAGE alerts and sounder auto-shut downs after I installed these chips that cost next to nothing.