GolfMikeMike - Similarly looking down on the Atlantic (North / South / Mid) from 35,000 feet I've never seen a single yacht, container ship, tanker or cruise ship - and only occasionally another aircraft
I fly over the approximate suspected crash sight regularly. I can also say I frequently see many aircraft (the airspace is busy), plenty of ships -both very large and normal size - and even the odd brave yacht.
You may not see comparatively very much because a) I enjoy looking out, but more likely b) the average sea state in the South China Sea is more benign than the North Atlantic you are used to.