Just a side note, my duck boat has 2 batteries but are separate size 24 deep cycle starting and size 27 for accessories for trolling motor, gps, and depth finder, and bilge pump and never thought about adding a switch and my 22 ft. Islander with 1 size 27 battery
handles all acc. easily with bilge (auto) ff/gps. Who really knows with a boat, both are setup by mfg. wiring in mind with no need to really need a change.

Gene