Commander William Marks from the US Seventh Fleet, which is taking part in the search, says he expects the plane's flight recorders to be floating in the water.

"In calm seas, if there were a soccer ball [football] or a basketball floating in the water, the radar could pick it up. They [flight recorders] typically have a radio beacon and so for example our P3 [radar] - if they are flying within a certain range of that - will pick up that radio beacon. We have not yet picked up anything, but that's typically what those black boxes contain."

I was under the (potentially incorrect) impression that flight recorders, by nature of the materials needed to protect their contents in the event of a crash, are quite dense and unable to float in water. I base this impression on news reporting of other airliner crashes (such as Air France 447, which crashed in the Atlantic), where the recorders were found some time later on the ocean floor.

Additionally, it seems reasonable to me that one would want to have the recorders not float so that they would come to rest near the other debris on the ocean floor since the recorders are equipped with underwater locator beacons which would aid in locating not only the recorders themselves but also the debris of the plane. If the recorders floated then they could move significant distances from the crash site and other debris, complicating search efforts.

It seems that Commander Marks, the writer of the article, or I (or some combination thereof) misunderstand something about flight recorders and their behavior in water.

My questions are these:

Do flight recorders float on water? If so, why?

I know that flight recorders are equipped with underwater locator beacons. Are they also equipped with radio beacons (either an active transmitter or a some passive device like a corner reflector to make them easier to detect with radar)?