Exploded transponders don't work. Radio waves don't really pierce water at all.

You're (as alot of other people) are grossly underestimating the problem of tracking things across oceans.

Not to mention the other simple fact: what's the point? Is there a difference if plane is found 1 day or 14 days after crash? Reliable tracking would be a huge investment into a system that at the end, wouldn't make much difference in survivability or cause of accidents.