What? You don't have headlights on your boat? No? Why not?
You should. An LED headlight like this one is dirt cheap (I got it at the hardware store for a few bucks). It serves to shine a lot of light on whatever you are working on - in the bilge, on the engine, or on deck. When you turn your head, the light follows, since it is right there, on your head. It's like you have a telepathic helper magically holding a flashlight, always just where you need it.
And this one ("Energizer", by brand) also has a red LED for illumination while preserving your night vision. Now really, you never know when this might become it's most important function.