I support the suggestion of a 360 degree white light, mounted on as high of a mast as you can manage on your kayak or canoe. Some sort of mast is necessary to prevent a substantial arc of the light from being blocked by your body. A mast will also amplify the apparent motion of the light. Consider backing this up with a very bright narrow beam flashlight, perhaps with a fast strobe function, that you can aim to get the attention of any potentially conflicting craft.