I think it has something to do with "Opening Wano's Borders." Because of the recent events, I think it's a safe bet to say that opening the Borders is much more than just proclaiming the Borders are now open. The Gorosei made it seem like when the borders are open, they could easily launch an assault on Wano. That means, the natural defenses of Wano will disappear. When Momonosuke told Zunesha that he won't be opening the borders yet, Zunesha left. So I think Zunesha is the one who will be able to open them. If that's the case, then I think it stands to reason, that Pluton is actually "sealed" somewhere on or around Wano, and impossible to get to, until the natural defenses of Wano disappear. Only then, can the ship be reached. Perhaps it's even submerged underwater, and when the weather of Wano becomes stable, the ship will rise to the surface.