A floating flight recorder will rapidly drift away from the aircraft wreckage and is more than likely to be run down and sunk by rescue craft or fishing boats attending the scene. The present system where if you find the tail, you will generally find the flight recorders works pretty well.
This appears to be more a case of what can you do to speed up the process of locating the wreckage in the first place in remote locations. The only sensible solution at present would seem to involve using a GPS locator beacon just transmitting a unique aircraft serial number every 15 minutes or so. The power requirements are low enough to be supplied directly from the aircraft batteries with circuit breakers and protection devices located away from areas accessible in flight. A beacon check before take off to check satisfactory operation could be verified by the control tower or ATC. No operating beacon, no flight.
The search area in event of a disappearance would still be potentially very large, but a plot of previous points along the flight path would narrow the initial search area considerably.