Good post from Paul M who by the large is correct. One small addendum...the piccie above is off a German Lynx which ditched in the Caribbean some years ago AND remained afloat long enough to be salvaged. Why? well it is generally considered that the main reason this one stayed on the surface was due to the cabin doors being shut and creating a bulkhead to ease the flow of water. It didn't make it watertight BUT it eased the flow and reduced the rocking.
I was always told that the main reason the Lynx floated like a brick was because of the nose. The area forward of the cockpit is empty except for a Radar scanner and once that cavernous area filled up with water the aircraft was only going to go in one direction.