Anything still floating after that impact would be indistinguishable from all the other floating garbage.
The plane is in Iran. The Chinese are in on the charade. They gave the nuclear technology to Iran. This Flight 370 was a test run to see if they can fly a 777 low enough so it is undetected by radar. Next there will be a 777 flying undetected into Israel, loaded with a nuke.
The 777 is made with the new composite material. If that 777 crashed in the ocean, it would have broken up. Its wings are composite. That composite material floats. The wings should have been found floating. All the seats float. If the passengers were still strapped in their seats, they would have been floating in their seats. If Flight 370 is in water, they would have found the debris by now.
Somebody else commented that the battery life isn't exactly 30 days. But a battery whose expiry date hasn't lapsed must not in the testing phase fail before 30 days. Good engineers always add a factor of safety margin to pass the tests so it would not be surprising that the pings continue for a few weeks or a month.
Both cockpit voice and flight data recorders with pingers are located in the tail which in the event of a crash are not subject to the higher g forces of the front of the plane. There is a maintenance data recorder in the cockpit area but it is not designed to be survivable nor does it have a locator pinger.
It is totally nonsensical that a non-Chinese airliner would land in Hainan (a Chinese island province) to off-load a secret cargo. It would not arouse any suspicion to offload this cargo at Beijing and put it on the next scheduled domestic flight to Hainan.
Why would you need an airliner the size of a Boeing 777 to carry a nuclear weapon? Nuclear weapons, no matter how crude, simply aren't very large objects.
Anything still floating after that impact would be indistinguishable from all the other floating garbage.