Tim I do not understand your comment "hemp, typical for the time..."
Typical of what time?
Nobody has dated this object.
If you look at common polyester rope, it looks very similar to this object. It appears silt/ mud or algae has filled in most of the valleys between twists, but on the still pictures the thread angle and spacing appears very similar to common rope, not cable.
The time - 1937.
I don't think polyester rope was available then.
I've never actually seen two-strand rope: please show an example if you can find one.
The line in question does not lie in silt. It lies on top of coral or coral-covered materials. Coral is abraisive, and judging by the channels adjacent to this line, this piece has been subject to much movement against the coral. So, therefore, I don't think there is any mud/silt/algae in any of the cracks.
These are my own opinions. YMMV.