Assuming a controlled ditching is possible with minimal airframe damage, the airframe could float a long time (after all, it's a lightweight structure that's designed to be airtight - Sully's A320 started sinking relatively quickly because they didn't have time to do the checklist which would have had them close the outflow valve).
Have you never seen photos of the aircraft after it was lifted from the Hudson? The lower rear fuselage showed major damage. Pressing the Ditching switch would not have helped very much.