Aircraft are quite bouyant and can float for a while. The RAF nimrod which ditched in the Moaray Forth in the late 1990s floated for some hours and a 707 which landed in lake Victoria floated for three days.
The ditching button will close all the holes on the 320, but the heavy engines should come off as well. The 'fuse pins' have a relatively weak breaking point in the event of a aftwards shock and they would normally come off on ditching. on Sully's aircraft, one came off. It's possible, the very low touchdown speed he acheived didn't rip both off. Also the high nose attitude meant the tail touched first casing the breach in the rear cabin.