From which direction must a white light be visible?

There have been a number of questions about boating lights here recently, so I'm assuming this is another.

The answer to this one is complicated, and depends on:

1. If the boat is a sailboat or a powerboat (sailboats which also have motors are considered powerboats if the motor is in use).
2. If the boat is in motion or at anchor.
3. The size of the boat.

You should really be a lot more specific with the question if you want a definitive answer; specifically you should say whether it's a sailboat or a powerboat, if it's under sail/power or at anchor, and how long the keel is.