Didn't I read somewhere that ETOPS was being reconsidered after several incidents in which all engines failed for reasons other than mechanical failure of the engines themselves? I think it had something to do with severe icing conditions that could stop all engines and had not previously been recognized as a hazard, but I don't remember the details.
Also, Sully wasn't the first. There was a Russian airliner that ditched on a river in the early '60s with all crew and passengers surviving.
From the stats in the Wikipedia article it looks like ditching overall is no more hazardous than an emergency landing on dry land. It seems that the landing surface isn't nearly as important as maintaining the aircraft in controlled flight until touchdown.