Has anyone got any ideas as to what this is?
Looks like a rope of some kind to my decidedly untrained eye.
We have to remember that Niku was an
inhabited island from 1939 to 1963.
It is visited routinely by both official and unofficial expeditions (including TIGHAR's, of course).
Many things could get dropped or thrown overboard by all kinds of boats near the reef.
The picture shows that there are "things down there" that might be worth looking at. We don't know the depth of this item, nor its size or length or composition.
I imagine that the Electra was equipped with tie-down ropes because I think I've seen other airplanes similarly equipped. I don't suppose that we can immediately rule out any rope as "not from the Electra," unless we can tell by visual inspection that it is too old or too modern.
From this standpoint, "what it is" is a reason to go do some more looking.