While I agree that preventing going over the side is the best avenue, there will be times where it is unavoidable. Broken tethers, not clipped in, abandon ship situation. etc.. Sailors are better indoctrinated on the use of safety harnesses, powerboaters are less so. I happen to own a power boat at the present moment and have no clip in points if I have to go on the foredeck. But this could also apply to dinghy sailors, sea kayakers, divers on charter boats. It is possible to get sufficiently far from shore that return is not possible (offshore breeze, high surf, currents (rip or otherwise)).

In that light, alert and locate equipment seems to be the priority. Flares (pen or mini), strobe, mirror, and whistle all would take up a sufficiently small volume and weight. A rescue streamer may be of use. When venturing offshore, one of the new personal EPIRBs or PLBs may be a viable suppliment, especially if the boat is equipped with a Radio Direction Finder (~$1000 USD).

Again, you have to wear it (have it) for it to be usefull, and the commercial version seems too bulky to promote continuous wear. Doug's answer using the Switlik vest seems better, but again may be too much for some users. A class IV (inflatable) with belt pouch may be more attractive to many, even if it does not provide any thermal insulation.

Does anyone know if any studies on the use of the equivalent of space bags (blankets) has ever been done for in water hypothermia prevention through the reduction in convective heat loss?