While it's true that you do get a break up on impact, the majority of the aircraft will sink rapidly, even if hitting reasonably flat, presuming its an uncontrolled impact. But obviously no one knows how it hit, if it was a nose straight down, very little would remain on the surface, even the broken up bits will be dragged down by the majority heavy parts.
In the unlikely event of a controlled landing on water, you'd expect the majority of bits to float, at least for a while, as the very heavy bits, the engines would most lightly sheer off on impact. I say this is less likely because if thats what happened, the crew would be shouting and screaming for help. They weren't, so it tends to point towards something catastrophic at height.