Extraordinary proof is the watershed, IMO, Bob.
Yes, it is merely an opinion, expressed in this form first by Carl Sagan.
It is not a self-evident principle, and it is not part of formal logic.
Extraordinary effort to overcome problems is applaudable - and my hat is off to that kind of spirit. But unless extraordinary success is attained in that effort to overcome, then no extraordinary proof is possible.
The history of jurisprudence suggests that evidence can meet the standard of "reasonable proof." But there is no formal definition of what is and is not an unreasonable set of expecations for such evidence. The kind of proof necessary varies with the case in question. The thought that
if the Electra landed on the reef,
then some large pieces of it should still exist is not a very extraordinary claim. It may be difficult or impossible to find those pieces, given the nature of the search area, but it is a project that has, in principle, a reasonable chance of success--or failure.