With no imperials in sight, you go to Isari to check on the wounded.
This quest is available if you have completed the quest The Lord of the Revel.
Starting the Quest
Talk to Lyse in the Ruby Sea to begin the quest.
Search for Villagers in Need of Aid
Go south to the larger orange circle on the map. Go to the west side of the circle and talk to the Flustered Fisher.
Go to the south edge of the orange circle and talk to the Stupefied Senior.
Speak with One of the Confederates
Go north to the small orange circle on the map. Go up to the top of the building in the southern part of the orange circle and talk to the Industrious Pirate.
Report to Alisaie
Go north to the stairs and make your way down. Go to the quest icon on the map and talk to Alisaie to complete the quest. There will be a cutscene.
Unlock Flying in The Ruby Sea
"Tide Goes In, Imperials Go Out" is the final quest for unlocking Aether Currents in the Ruby Sea. If you have completed the other Aether Current quests and attuned to the Aether Currents in the field in this area (which you can find using the Aether Compass in Main Menu > Duty > Collection in this area) you will be able to fly in the Ruby Sea.
Next Main Scenario Quest
After you complete the quest, talk to Gosetsu to accept the next quest: A Silence in Three Parts.