A fellow on a radio show I listened to this morning said the black boxes have about a 3-5 mile transmission radius, and will only transmit for about 30-45 days on their battery power. He (nor I) was not sure the depth of ocean the waters in that region. As a comparison, he did mention that they only found the black boxes of the Air France crash after finding some debris. The transmitter had ceased operating long before they located the wreckage. (They got lucky to find it - or maybe more accurately, just kept at it until they did)
what if it is not where they are looking.
what if there was decompression and everyone was knocked out by sudden air loss?