This page explains how to complete the level 80 post-Shadowbringers quest "On Rough Seas".
This quest is available if you have completed the quest The Search for Sicard.
Starting the Quest
Talk to Alphinaud in Lower La Noscea to begin the quest.
Speak with Merlwyb in the Command Room
You can either teleport to Limsa Lominsa or you can go southwest to the gate into Limsa Lominsa. Go east from the Limsa Lominsa Aetheryte Plaza to where the orange arrow is pointing in the minimap. Talk to Zanthael and say Yes, then talk to Merlwyb to proceed. There will be a cutscene.
Speak with Merlwyb
Talk to Merlwyb to complete the quest.
Next Main Scenario Quest
After you complete the quest, talk to Merlwyb to accept the next quest: The Great Ship Vylbrand. There will be a cutscene.