It’s One Piece
I think the whole “Fishmen are vile creatures who don’t deserve to live on the surface” is a nod to how the real world is. Some people dislike others solely for the way they look, rather than for who they are.
If Fishmen didn’t do anything wrong, then they should be able to live on the surface and see the sky along with its stars.
It must feel terrible being under the sea all the time, not being able to go above without people treating you terribly...
Fisher Tiger and Otohime‘s story helps to develop this dream of being able to live amongst those who reside in the world above.