On the two HUET courses I've done (one at Sale in Vic, Aus, and one run by the Royal Navy in th UK) we were told that if the floats work, it'll stay upright and you can get out (sea state permitting).
If the floats don't work/are not fitted to aircraft/high sea state, it will roll over and either float belly up or sink, either way you'll need the HUET training.
The pictures etc that we were shown at the naval dunker (to keep us amused until it was our turn), showed a lot of belly up aircraft.
I think Ascend Charlie has it spot on with the heavy bits at the top, and light/empty bits at the bottom. Not sure how it would work for Wessex style aircraft, with the engine down low in the nose, and some transmission up top.