Don't the black boxes send out some sort of beacon when they hit water?
Yes. And the Vietnamese have already indicated as much I believe.
At this point, the Chinese, the Malaysians, the Philippines, and the Vietnamese are all sending vessels and aircraft to do the search.
The South China Sea is fairly expansive, but mostly not too deep. I believe thee weather was not bad and if it remains that way, they will begin finding more debris soon.
If there was a catastrophic failure at that altitude (and I believe current evidence points to that), as one poster indicated, the debris field is going to be large. but the black boxes for avionics and the cockpit voice recorder were designed to survive such things an when those are found, the investigators will be able to determine with fair certainty what caused the crash.
Condolensces to all of the families, friends, and loved ones of the deceased...and may God rest the departed.