Dave-
I've seen this done by taking the bilge pump to a 2-way switch on the breaker panel (one way for automatic, one for manual, sometimes centeroff as well) and an extra "master" switch on the panel, so that everything can be off-- if you hit the master. Or else you leave themaster switch on, turn off everything except the bilge pump (where "off"really is the automatic position, so the switch looks "off" like everything else) and the battery a/b switch is normally left ON in order to supply power. I think it is a matter of convenience...but on a boat where gasoline or propane is used, you really want to make sure that when the main battery switch is OFF everything is off, nothing by passes it--because someone else may be minding that one day, and not knowing you have wired the bilge pump independently.