Al - this may be an impertinent comment but, before you start, be sure you can do the joining properly. Salt water travels a long way up any cable turning the wire black and corroding it away in time. It's always best to avoid using deck plugs. Use a cable gland instead and make the joint somewhere you hope will stay dry. then wrap up the joint in self amalgamating tape. Many plugs require good eyesight and a very fine soldering iron.
You can certainly rejoin the cable but as said previously it's best to make the connection well inside the cabin. I did this three years ago by twisting the wires together and wrapping each with insulating tape.It is still working normally.
The wires are very thin so do take care and finally do remember to reconnect the copper sheathing as this acts as an earth.