Luffy explained that he will make Fishman Island his territory at some point in the future, showing intrest in having a place for his crew and him to stay safely.
And since it’s pretty much a fact that he will, I don’t know in what context since it has been prophesied by Madam Shyarly that someone bearing a straw hat will destroy the island.
Wano is perfectly set up for a Fishmen and Samurai coexistence. Here’s why:
- Wano’s geographic characteristics. Many rivers flow through Wano, making it easily accesable for Fishmen. Giving them the chance to live on land if they want (it has been a dream of Neptun’es wife to live among humans). Then there’s Onigashima offering the option of a palace for royal Fishmen.
- After the tyranny of Kaido comes to an end by the hands of outsiders, Wano Country won’t have the same inclination towards resenting outsiders in the first place, making it more likely to be accepting of refugees.
So when push comes to shove the Fishmen will travel on the Noah towards Wano, on behalf of Luffy’s/ Jinbe’s invitation.
*Theory by rustynailclipper