Don't buy cheap spotlights or flashlights for use on your boat. In an emergency you need the best, and you need to have them placed where you can grab them instantly. And don't go for the "million candlepower" claims that you see on brightly colored boxes. There are now very specific standards developed by responsible entities in the flash/spot light industry that can help you to judge what you’re getting. See the article by Tom Neale to learn about those standards. They will help save you money and maybe your boat and your life.
Look at the standards values on the box first, then consider other things having to do with your use. You'll want a durable, long-lasting light, and the quality of construction is a major factor in the reliability of a spotlight. Look for booted switches, stainless steel hardware, rubberizing to minimize shock and vibration, and waterproof, or at least water-resistant, construction.
Ergonomics, the comfort and ease of handling, also plays a role. In any emergency, the ease of one-handed operation may be important. Pistol grips are generally the easiest to use — some lights are much more comfortable to hold for extended periods than others.
When using a spotlight, remember that the human eye requires from 12-40 minutes to fully recover its night vision after exposure to bright light. When using a spotlight on the water, point the beam down low, or in front of or behind another boat, or even up into the rigging. Avoid directing your beam in anyone's eyes — you don't want to destroy the night vision of another boater trying to operate safely in the same area as you.