For $24.46 on ebay, shipping included, they have a bright white l.e.d. light with on/off/strobe setting, powered by three AAA batteries, made by Propel Paddle Gear. It comes with a mount base that gets screwed to the small, non-motorized vessel and can be removed from the mount and waved around by hand.
http://www.amazon.com/Shoreline-Mari.../dp/B017P3AYKA .... check out the customer reviews down the bottom.
On Saturday November 9, 2019 at about 6-pm, two guys, ages 76 and 84 in two boats, both about 18-20' length motorboats both got killed in a boat collision at the north end of Bear Island presumably due to not seeing the other boat in the dark and colliding.
On a kayak or paddle board out in the dark, you could definitely hear an approaching motorboat boat as well as probably see it. The BIG question there is will the approaching motorboat see you!