Have they determined if the oil slick is fuel dumped from the Malaysia Jet? That would indicate the pilots knew they were in trouble & planning a hard landing (or a skilled Sully) - but to not report a problem is very strange. Too many abnormal circumstances to be considered a ‘routine’ accident.
I don’t know about the slick. But a simple chemical test should tell whether it’s from a plane or a ship. But, assuming they weren’t hijacked, it’s unlikely they made a leisurely water landing without notifying anybody. My guess is they went in fast and at a sharp angle. That might explain why they didn’t have time to radio their condition.