Using a seperate electronics battery main advantage would be elimination of the possibilty of interference.
A smaller battery Amp/Hours rated can be used as current draw would be low as compared to main system battery If you have a extra deep cycle laying around use it. But if you have to go purchase one I would get the smallest and least expensive one.
Even a regular marine battery would work. Just don't use a type that is not designed to be constantly discharged and recharged. Like a car battery.
Garminarmin gpsmap 1040 xs networked with Garmin gpsmap 1242 xsv , Panoptix PS 30 and PS 31. Livescope LVS 32
Ice rig Echomap Ultra 106SV with LVS 12 and GT10-IF