I read in the initial reports on the Hudson ditching that the crew did not do the checklist (for good reasons) and thus did not activate the Ditching Switch. I guess that switch closes the various valves in the fuselage and stops water from entering, extending the float time. It also appeared that with the valves still open, the water entered so rapidly that it tore the bottom out of the lower rear fuselage area, hastening the sinking and preventing the rear doors from being used. If the Ditching Switch had been operated, maybe the airplane would have floated better?
Probably not, but who knows?