120 v ac

[Charlie]

I found some number 12 wires tucked away in my lockers. They had once been used for getting AC power from an inverter to run a washer. I want to use them to put a second circuit on the port side of the boat.

From my residential experience I want to splice the wires in a box. But I realize I would be using heat shrink butt splices instead of wire nuts in a box.

What is kosher? Box or just butt splices.

[Wotname]

Box is better (and perhaps required) for 120VAC.

[Coquina]

Can we assume these are marine grade tinned stranded wires? You can join them with crimped butt splices or use ring fittings and a terminal strip. Do NOT use home style wire nuts on a boat EVER.
I would definitely have this happen inside a junction box and have the wiring secured so you can't yank on the connections.