Somewhere in the Atlantic, there is a lighted bouy blinking on and off as you flip that switch.
Assumption is that because it is close to the AC panel, it has something to do with AC...
The two most common AC "options" are (as others have already pointed out); Battery Charger, and Water Heater. Because there is a light on it, I would guess Battery Charger.
However, because the boat looks to have some, uh... non-standard wiring, it could control virtually anything.
If it were my boat, I would be rewiring EVERYTHING connected to this panel with tinned - stranded - Marine grade AC wire.