It is an evacuation slide or slides. Do a Google search on “777 evacuation slides”. Although the dimensions given to the news media, 20m x 13m seem too large, they could also be in error per the rest of the data Malyasia gives out. The slide from a distance would appear as an “extended U”. The 777 evacution slide photos on google show them to be about 20m x 5m. The “relative dimensions” as seen on the Chinese photo show it to be a slide. If it is a slide there could have been survivors.
The only problem I see with that and survivors is I believe those slides come with some form of communication equipment or distress beacons, even if they are empty they should still have activated radio beacons.
I looks like it could be a catamaran to me, about the right dimensions for it.