It's a three pole, shore power, selector switch for a Grand Banks 49 classic, 1983, that bit the dust.
Tough to find the exact switch.
Anyone have one collecting dust on a shelf?
The label is upside down in the photo so I can't read it but Grand Banks probably didn't manufacture the switch, they probably bought it and installed it.
Look for the manufacturer's name and the model number on the switch and then do a web search to find a replacement.