ishoot has it right.
There are no spotlight specific rules. But there are rules that state what lights can be on during navigation, or while anchor/mooring, and a spotlight is NOT one that can be on (but they can be used momentarily).
That being said, the best spotlights will have a 'trigger' switch, in that you can turn on the spot for a short blast as needed. This solves many problems, one of which is preserving your own night vision (while also not blinding other boaters, and not allowing them to see your navigation lights because your spot/docking lights are on).
I used to think that the best spot must be the powerful one mounted permanantly on my bow. It has both a flood and spot mode and I can point it remotely from my helm. All larger boats seem to have these, so they must be the thing to use, right?!?! NO!
I had an experienced night boater go out with me and explain why these things are rarely useful and could be dangerous The problem is that they are too slow to move and hard to get on target to illuminate a potential hazard or a navaid, etc.
The best thing to use is a hand held spot with a momentary on switch. This way you can point it naturally and quickly with your hand, and then turn on the light just when you need it to momentarily illuminate a marker, dock, boat, etc.
If you tried to do this with a boat mounted remote spot, it would be on for much too long and it's too hard to use to accurately point while you and your boat are moving.
I think the best hand held spots can run both from a power jack on your dashboard, and also can be run from an internal battery. This way you have a sort of a backup. If the battery is dead, use the dashboard power. If the dashboard connector fails or a fuse blows at night (I've had it happen), you can use the internal battery (always make sure it is fully charged before your night navigation starts). If you are the boy-scout or over paranoid type, have a backup spotlight available too as the bulbs are very fragile if the spot is bumped while the bulb is still hot.
I'm not sure if the new LED spot lights are as powerful as the halogen spots yet. Perhaps the newer, more expensive LED spotlights are good now? It's hard to compare them as the LEDs are rated in 'Lumens' and the non-LEDs are rated in Candlepower. I find my 1,000,000 candle power spotlight is just barely adequate at times (it has a 100 watt halogen bulb). Here's an interesting article or two if you are trying to compare Lumens to Candlepower:
http://www.flashlightuniversity.com/...s-candlepower/
http://www.ehow.com/how_5997091_conv...er-lumens.html