A C130 hercules (Columbian Airforce) ditched in the Caribean and floated for 24 hours. It didn't sink till they tried to tow it to the coast.
A 707 ditched in Lake Victoria, Tanzania and everyone got out. It was then towed to the shore and sat on a sandbar.
The problem with ditching is not whether the aircraft will float but the state of the emergency that causes it to be ditched in the first place.
As for the recent water crashes, none of these were an attempt to ditch the aircraft.