Found the problem. It was the connection between the bow light and the main circuit. I found an inspection port in the anchor locker through which I could touch the connection. When I wiggled it the light came on and off. Unfortunately I could not reach the connection well enough to repair it. I was able to create a new connection elsewhere on the same circuit, though, and cut and insulated the wires to the old connection. The bow light now works. Onward to the non-working mast lights!