My grandfather was the smartest man I knew. He could build or fix anything. He was 17 when the stock market crashed in 1929. He quickly learned the value of the dollar and materials.
His bench grinder was an old Maytag electric motor out of a washing machine he rigged up the spin a pair of stones. He build his own tractor from the rear end of a WW1 truck, some steel beams, and an engine and transmission out of a model T. One of the cool things he build was a "Continuity Tester" out of some porcelain lamp holders, a board, and some wire.
He was using it to check some circuits on the travel trailer while I was hanging out with him one day. To an 8 year old watching the lights switch on and off while he was touching the bare wires to the trailer wires was pretty neat. He went into the house to get something and I wanted to make the lights turn on and off too. By the way, this contraption was plunged into the house 120 volt circuit.
The ensuing flash and sparks scared the crap out of me and sent grandpa fly out of the house to the trailer. He was a very patient man and did not get upset easily. He ask me if I was okay explained what happened when I touched the wire to the hot side of the porcelain lamp holder. He unplugged the rig, went in and turned the breaker back on, then removed what was left of the melted brass screw.
That was my first bad experience with electricity. After owning boats and working on hotels I've had so many more. Electricity is nothing to take lightly. Even low voltages can cause fire, hurt or kill you.
This is the creative wiring under the helm in the flybridge of our latest boat. We found wire nuts, connections that were held with electrical tape, loose butt connections and bunches of end ends. By far the best was the power strip. Someone had put a metal bar on the breaker panel buss in the solon and wired this thing in. There was no circuit breaker on the panel protecting this, only its internal breaker.
On July 12th 2020 in Peoria Arizona two brothers lost their lives by jumping into the water next to their family boat while at the marina. Investigators found a modification to the shore power cable system which energized the boats underwater metals.
American Boat and Yacht Council, ABYC has a set of standards for how boat wiring is to be installed. You can find their webpage here. There are also links to Certified Marine Technicians, Marine Surveyors, and some good DIY information on their page.
Consider adding an Equipment Leakage Circuit Interrupter, ELIC to your A/C electrical system if your boat is pre 2013. This interrupter is designed to prevent Electric Shock Drowning.