Black boxes: great for crashes on land. How about additional "floaties" for water crashes (low-tech, light, cheap)
The first thing that comes to mind to me is that they'd have to be firmly attached enough to not come off in flight, yet somehow detach easily when in the water. They'd also create additional drag, which - especially to commercial air carriers - translates to money lost (additional fuel). 

There would also need to be some connection (wired or wireless) to continuously transmit data to them (I'm assuming that you're proposing several of them). 

Finally, the whole point of the FDR's housing is that it's rigid enough to withstand explosions and impact, yet the floatation proposal requires that there be some sort of expanding material that'd inflate upon contact (or presumably submersion - consider weather/rain) with water.

General aviation aircraft use ELTs (Emergency Locator Transmitters), which are designed just to notify of the location of a crash incident. They are required in the U.S. for general aviation and commercial aircraft, but unfortunately not for scheduled flights by an air carriers.

I get the impression that the trend is probably towards having more data transmitted real-time during the flight (think Air France stall/crash) of systems monitoring data and location such that location and recovery of more data-intensive logs (voice recorder) would be more efficient.