Even if it were water proof, it would only be so at a certain depth. The pressure asserted by the ocean would render it useless...and the density of the ocean above it would probably make the signal too weak.
While that's true at the very least (assuming that there really is a cheap worldwide GPS tracking available) the last known position would be much more accurate than in MH370.
That would make unnecessary the attempt to extrapolate the existing data (the pings) in order to infer the plane last location good location.
