User blog comment:Kunoichi101/Chapter 958 Bad Omens in Wano/@comment-25558176-20191004095747

This is my prediction; The boats are missing from the harbor because they are in the air.

There was a panel from a chapter a while back that showed us small boats that were tied to balloons and sent into the sky to show respect/mourn the dead during some sort of festival.

I think that panel was foreshadowing something, and after reading this chapter, I'm more inclined to believe that this is what it is.