In seriousness, I'm wondering why the trip took 25 days. Flying over the area would likely be more (of) a reasonable option.
Unless you actually wanted to sail around the south pacific for a month and be able to call it "work"
Edit: The initial "nonsense verse" I used made me curious so I looked it up and that's where I found the ending of one version of the phrase.
"I see", said the blind man to his deaf wife as their dog with no legs got up and ran away. He pissed in the wind and said "It all comes back to me now."
In other words, they were seeking to establish that it's not an island that once existed and has been submerged due to rising sea levels, it flat-out never existed.
Not sure if it's possible (or at least, best practice) to do that from an airplane?
If this were the case then depth soundings would easily identify a submerged volcano, even if it were 100m below the surface. A simple visual fly-by wouldn't.
Or, y'know go out there to map the place to get better and more accurate maps.
If it has an island that isn't there, it's not a good sign that your maps of the area are accurate.