Hirase needs you to help cull the ravenous sharks that keep eating the Confederates' catch.
This quest is available if you have completed the following quests:
Starting the Quest
Talk to Hirase in Onokoro in the Ruby Sea to begin the quest.
Take the Rotting Fish
Check on the sparkling Sun-ripened Chum.
Use the Rotting Fish
Go east to the orange circle on the map.
Go to the western edge of the circle. Dismount and target the purple Destination, and then you can either examine or target the Destination and use the Rotting Fish on it from the Key Items section of your inventory, or you can target it and use the icon to the right of the quest name in your list of active quests. Kill the Aggressive Shark that attacks.
Then go to the eastern edge of the circle and do the same.
Search for the Industrious Pirate
Go east to where the orange arrow is pointing in the minimap. Dismount, enter the purple circle, and kill Sharpshell of the Red.
Rescue the Industrious Pirate
Check on the Industrious Pirate.
Report to Alisaie
You can either teleport to Onokoro or just go west to where the orange arrow is pointing in the minimap. Go up the wooden stairs until you reach the quest icon. Talk to Alisaie to proceed.
Speak with Rasho
Go up the stairs until you reach the quest icon. Talk to Rasho to complete the quest. There will be a cutscene.
Aether Current Quest
With the completion of this quest, you can now start the blue quest "Pulling Double Booty" in the Ruby Sea. Completing that quest will attune you to an Aether Current in this area.
Next Main Scenario Quest
After you complete the quest, talk to Rasho to accept the next quest: Under the Sea.