Water plus darkness is pretty much no one's idea of a good time. Should you find yourself alone in the water in the dark, a cello playing ominously in the background, you'll want someone to find you and fast. Whether it's on land or water, you can help them help you with a signalling device, and it's hard to beat the EF-20A-1 Lightning electronic flare from North American Survival Systems.
The Lightning is housed in a sturdy aluminum cylinder with a tough polycarbonate dome on the business end, and a lanyard on the tailcap. It's under five inches long, 1.25" in diameter at the light, and weighs five ounces with the included two CF123 batteries. It's four CREE LEDs are visible up to ten nautical miles away at night and 1nm in daylight, with 360 degree coverage. Operation is stupid simple, the only kind that works in stressful situations: push the recessed tailcap button once to turn on, once to turn off.
The Lightning flashes once per second after activation, and will continue strobing for 35 hours. Unlike a conventional flare, it won't burn you or anything near you that might be flammable. It doesn't float, but it is waterproof to 50 feet. The Lightning is aptly named: if you've got one of these on your person and it's visible and operating, your chances of being seen are good. Highly recommended for any activity where you need to be seen.