Can any one advise what this bottom switch is for ?
Gb42 #1386
Photo is dim and grainy, but I think it's the load switch for the ammeter. At least it is for my 2000 GB42. Do you have the GB manual that came with the boat? It has some details in it, plus some line drawings of the electrical systems. I believe it measures the amps on either L1 or L2. My AC system is on one "leg" and all other 120 V items are on the other leg.
I am always amazed at how little importance previous owners pay to proper labeling of switches and buttons. "Well, I know what that does; so why bother to label it?" It does not compute with me.