Another thing to bear in mind (and something most people forget) when dealing with 12V circuits on a boat is that batteries can deliver *large* amounts of current under short-circuit conditions - many hundreds of amps for a very short time. This is far more than you'll ever get out of a shore power connection without tripping something on-shore first.
Apart from the risk of fire on board from melting wire insulation (the obvious one), a more insidious problem is that high DC currents cause magnetic fields that will tend to push +ve and -ve wires apart (usually seen as "wire jump"). These forces are pretty impressive and can crack bulkheads wires pass through, force wires partly out of terminal blocks and damage the batteries themselves, so it's wise to check the entire run back to the battery if ever you accidentally short something out.
To stop this, a good practice is to fit a fuse or circuit breaker to the
battery itself (like those fitted to cars for the same reason). These fit on one of the battery terminals and, rated to the maximum current of your house power, will limit the amount of accidental welding you'll do whilst fiddling with the wiring with the power switched on...
