Source: AP forums

Human pirates are released and Neptune says that the remnants of the NFP can live on the main island as manual laborers.
Fishman district is blocked off. The Officers, Decken, and Hody are imprisoned. Wadatsumi is effectively deported and taken to a far away part of the sea by the Kraken.
Jinbe refuses but he says to wait, because he has to remain faithful to his duty. He says to come again another time and try inviting him once more.
The party on the island begins.

Robin sneaks away from the party and goes to see Neptune (just the two of them).
Robin asks about Joy Boy. He was a real person who lived in the void century, and that apology he wrote was aimed at the Mermaid Princess at the time.
According to the legend passed down by the royal line, it seems that someone would someday appear to fulfill the promise in lieu of Joy Boy. There was the promise to protect Noah until that day finally came.
On Sky Island, Robin knew the whereabouts of Poseidon. Poseidon was on Fishman Island, because Poseidon IS Shirahoshi, the mermaid princess.