Pages 12-13 of Ch. 435 introduce the Florian Triangle, a cursed sea on the route to Fish-Man Island: ~100 ships vanish yearly, crews disappear, and eyewitnesses report ghost ships crewed by the dead (a skeleton-crew panel is shown, p. 13).
The payoff
Earliest known caller
(Unrecorded.)
Why this matters
A chekhov-explicit + background-symbol beat: Oda names the cursed sea, shows a skeleton-crewed ghost ship in the same scene, and has the captain voice the exact wish (“a living skeleton”) the next arc delivers — the seed and its register are planted one chapter before the crew sails in. Cross-link Florian Triangle, Luffy, Brook, Fish-Man Island Exists Mermaids Live There.