I note from Macarthur Job's "Air Disaster" Vol 1, pp67-73 than an Overseas National Airways DC9-33F, N935F, ditched and retained enough structural integrity for 35 of the 63 onboard to survive and be rescued. How many were killed on impact and how many died of exposure post-impact is unknown.
The a/c stayed afloat for about 10 mins. This is apparently one of very few successful ditchings of a commercial airliner. I note also that the DC9 wing is a clean wing, bereft of engine-pods and notably strongly built. There was speculation that the fuselage underside was breached but the water depth was 5000 feet and recovery of the wreckage was not contemplated. The reason was not in dispute, anyway - fuel starvation.