After the city is restored you go to the grand talk to Ame and then Adrienn infront of his gym in Coral, Victoria in Berly Graveyard and Arclight near the Club in North Obsidia.
Talk to Ame and do what she tells you.
As for the unterwater houses, there are some open already inside of them you can find hidden things and the keys for other houses. Not all can be opened.