Any photographer (land or underwater) will tell you that the key to a good photo, is good lighting. You can never have enough! This is even more important for underwater photography, as the water absorbs the light so much more than it does on land.

Many photographers opt for a combination of strobes and lights, as strobes are significantly brighter and more powerful than your standard light, and they do a much better job at freezing the motion. You can get away with very slow shutter speeds, while not having to worry too much about blur or camera shake.

Strobes can be a significant investment, so most beginners start with a good photo/video light. You want something with a fairly wide beam (to avoid hot-spots in your photos), and the brighter the better! The advantage of shooting with video lights is that you know exactly what the lighting is going to look like before you take your picture. Not to mention they are a lot smaller and less expensive than strobes.

Having a light will also help your camera focus a lot easier!