THE RULES SAY...
that the lights are required items, that you can't have "pretty near" or "work around", especially underway. There are portable ANCHOR lights made by Davis that are great, legal, use very little 12V, plug into the cigarette lighter on a long cord, about $35. But its not a steaming light. If you're ever under power at night, you need a light on the mast, above all other lights, with 225 degree coverage on the front of the boat, with 2NM visibility. Anything different, and after a collision, YOU'RE LIABLE. If by "mast light" you mean an anchor light, it has to be 360 degree visibility, but same other requirements. (This is the Davis ligt referred to.) I've known people who owned boats not displaying a legal anchor light who were hit at 1:30AM by a drunk on a jet ski (also with no lights!), and were partially liable. If you mean a deck light shining down, there's no legal requirement, just convenience.