And how many pax are going to panic and inflate right when they see water, or maybe even before. I'd just hope they get low bouyance units.
Frankly I think far more folks have drown INSIDE the plane than once outside the aircraft.
Where I fly mine, if I ever biffed a landing I'm right next to the shore or shallows anyways, I'm not worried about mortality and morbidity once outside of the plane, it's the getting outside that's the issue.