I think Scott has it right. The 72 COLREGS (international rules to avoid
collisions) states that kayaks can either use the same light pattern as a small
sailboat (red/green) or just a white light that can be shown to avoid a
collision. The picture in the regulation shows a sailor using the light to
shine on their sail to make them more visible.

You may also be required to have a night visual distress signal. The most
common for kayakers would be three aerial flares (three count as one device) or
a strobe. According to international rules, the strobe must automatically
flash SOS in Morse Code. Fortunately with tiny computers, you an get a light
to do that for about $35US.

Hope this helps.
Brian Blankinship