If the hull is intact, and the pilot can glide it in, then, yes, a plane can float briefly on water, Airbus even builds in a special ONLY IN DITCHING SITUATIONS button that basically closes all the ports to keep the hull as airtight as possible if it ditches. We saw that with Captain Sully on the Hudson. But this is the extremely rare scenario. In fact, Captain Sully is the only occurrence I can think where a jetliner landed on the water and everyone got out like that.
But if a plane breaks up in the air, we're talking about billions of pieces floating down to the sea. Or, if it slams in the ocean intact, it's still going to shatter in a billion pieces. Remember, water doesn't compress. Slamming into water at high speed is essentially no different than slamming into concrete. The latter is what happened to that Air France flight about 6-7 years ago. If you want to be terrified, read the detailed articles about that. The senior pilot took a break, so the co-pilot and a junior pilot were at the helm. They ran into a rare weather pattern that essentially froze the airspeed tubes, so the autopilot lost sensor data as to how fast the plane was going and shut off. The co-pilot and junior pilot suddenly find themselves in manual control, and they're trying to find out why their altitude is suddenly dropping. All this time, the computer is telling them "STALL STALL STALL." Now, if you know anything about flying, you know that if you're in a stall, you have to get the nose pointed down to pick up airspeed, because you need air flowing over the wings to generate lift. The co-pilot was pointing his control stick down to go into a dive, but the junior pilot panicked and fucking yanked his stick up, which is something someone who isn't a pilot would do, because up means up, right? The damning thing is that they weren't really communicating this fact to each other, and the flight computer basically took in their opposite commands and neutralized them, but without telling them, either.
It wasn't until the senior pilot got back to the cockpit in an alarm and asked what's happening that the junior pilot said, "I don't know, I've been pulling up this entire time!" The senior pilot and co-pilot started screaming "NO NO NO NO" at the junior pilot, but it was too late. And that's how two pilots unwittingly flew a perfectly functioning jetliner straight into the ocean, killing everyone. The plane shattered on impact.
The morbid joke and reality is that all those tiny life preservers and seat cushions are for is to create flotsam on the surface that helps searchers find where the bodies are at.
We'll have to see the state of the plane before we can figure out what happened. The fact that they haven't found debris yet doesn't mean anything. Even the South China Sea is a big-ass body of water when your'e trying to find something.