Difficult, yes. But if you are held out of the water, especially if there is a warmth-conserving wrap around oneself, you could survive indefinitely I should think.

We have a good system of alerts on boats and planes and there are tons of people who go hunting whenever someone is missing, so a brightly colored object floating in the water has a fairly good chance of being spotted. Around here, boats, canoes, kayaks all have bright bottoms.

My original question remains: Does anyone know if there is already such a device?