Something that does not seem to be in the ATSB report us an analysis of civilian shipping location and course at the time of the 7th partial handshake. The information we do have is the exact time of this transmission, and the location of this arc. The location of ships in the vicinity of this arc at that time is surely available.
I might be totally wrong but... Although ships are required to keep adequate lookout I don't think this means they are required to have a man physically staring out to sea all the time. I believe the legal term "lookout" refers to an overall "information collecting process" that can include staring at radar. I can well imagine that particularly at night that's all they do.
So unless the aircraft appeared and was recorded on radar as it impacted I doubt anyone saw anything. Might be different if it was on fire though?