Why bother lugging something else around? Design & install a reliable electrical system, house bank and reserve, and leave the extra stuff at the dock.
Come on! Have a little imagination - it's a boat, things go wrong all the time, even when you've prepared properly. Ever carry a rigging knife on you when you sail? Besides, it's the size of a thin paper back book - for that kind of insurance (and if it works) I'll make room by taking out one of the 10 hand towels my wife keeps in the head and tuck it away there.
Even if
we didn't need it, might be nice to lend a hand to someone who didn't design & install a reliable electrical system, house bank and reserve, and left the extra stuff at the dock.