This page explains how to complete the Shadowbringers Main Scenario Quest "To Storm-tossed Seas".
This quest is available if you have completed the quest The Unbroken Thread.
Starting the Quest
Talk to Urianger in the Crystarium to begin the quest.
Speak with Urianger in Sullen
You can either Teleport to Fort Jobb or you can use the nearby Aethernet Shard to go to Tessellation (Lakeland). Go southwest to where the orange arrow is pointing in the minimap. Talk to Urianger to proceed.
Investigate Suspect Rock Formations
Go west to where the orange arrows are pointing in the minimap. Go to the easternmost orange circle and dive underwater by pressing Ctrl+Spacebar on keyboard or by pressing B on Gamepad. Go just east of the center of this orange circle and go to a depth of Z: -0.3 (check below the minimap). Check on the sparkling Suspect Rock Formation.
Go west to the other orange circle on the map and go just northwest of the center of this orange circle. Go to a depth of Z: -0.5 and check on the sparkling Suspect Rock Formation.
Speak with Urianger
Go upward to go above water. If you have unlocked flying in Lakeland, fly to the quest icon on the map. If you have not unlocked flying in Lakeland, go to the northeast side of this island to find Alphinaud. You can walk up onto the island's surface there. Go west to the quest icon. Talk to Urianger to proceed. There will be a cutscene.
Speak with Urianger Again
Talk to Urianger to complete the quest.
Next Main Scenario Quest
After you complete the quest, talk to Thancred to accept the next quest: Waiting in the Depths.