The drawing above is for a
24 volt system. The output is 24 volts. There is no 12 volt output shown. Your decision to seek professional advice is a good one.
If your boat stays IN The Water at a dock with shore power, it is IMPERATIVE that the wiring in the boat be correct, or you risk "electrolysis" which will quite likely EAT up your outdrive, or outboard lower unit if it remains in contact/imersed in the water. This damage can actually happen
over night (while plugged in to shore power) and will be seen as blistering and shedding of the paint on the lower unit. The aluminum under the paint actually disolves.
As an aside: This same result can occur if the Dock is incorrectly wired.
NB