It sounds like someone just stuck on the closest labels they could find. Your "masthead" switch could be for a steaming light (as Cam mentions) but some boats mount a combination steaming/deck light on the forward side of the mast. This is one fixture about the size of a beer can, with two bulbs and sections in it. One to shine down on your deck, the other a steaming light. They are usually controlled by two switches (although one 2-way switch and some diodes can also be used).
Bulbs burn out, and sockets corrode. Could be the switches are fine but the bulbs burned out, or there are other problems. When you replace the bulb, clean the socket and put some silicon grease in it to prevent future problems.