Does it transmit from the ocean floor (radio signal)?
It does and it will.
How hard? I do not know.
It would certainly depend on the depth underneath the wreckage site.
Crash survivable boxes are not flotation devices. They will be with the wreckage.
The ocean floor where they are looking is 13000 ft deep and the black box emits a signal for about 30 days-have to see if they can find it.
The black box is in about 2000 to 3000 meters of water. It will not be easy to get them(there are two I believe).