do floatplanes ever sink after capsizing after ditching after engine failure?
Most certainly they do. I saw a programme on Discovery Wings (or something like it) a few weeks back that consisted almost entirely of film of floatplanes crashing, breaking up and sinking.
Actually, when you think about it, even boatsmen wear lifejackets and ships have lifeboats, and these are machines designed to float, not fly!
Seriously, a floatplane stands very little chance of landing and staying in one piece in some of the swells that are common in the North Sea and English Channel, and, as you are
more likely to fly a floatplane near water, there is
more reason rather than less to carry a dinghy.
IMHO.
Timothy