We all gotta learn sometime ;-)! But seriously, it sounds as if you two need to go back to the very basics. 12V electrical systems in boats are about a simple as electrical systems come, but their design, construction, installation
is not taught in school
. Tho' by my lights, it should be :-)!!
and look around. It's only a starting point, but as with all things in life, you gotta learn "from the bottom up". And because "the devil is in the details", it never hurts to review something you already know.
You haven't said what sort of boat you have, but in terms of electrical systems, few are simpler than those of a small sailboat. The systems of bigger boats are just "more of the same": Same fundamental principles, just more circuits and more boughten devises, the inner functions of which you can learn about later.
Do yourselves a favour that will conduce enormously to your understanding of these things and will furnish you with an implicit list of "do"s and "dont's" that will preclude these kinds of things happening in the future. Draw out on paper a diagram (among the cognoscenti called a "schematic") of your boat's wiring. Then sit down and do a systematic "study" of how the basic - really, really basic - Theory of Electricity relates to each component of that schematic, both in terms of the sundry boughten devices and of the wiring that connects them.
We are here to help, but we cannot really do that until you give us SPECIFIC information ("basic specs" as given by the manufacturer) of the devices you want help with. It is also useful for us to know the make and model of the boat you have.
Remember that none of us were born knowing these things. You are not "at fault" for not knowing. The public education system is at fault for not having taught them :-)
All the best