The Last Wish - Skellige
This quest will automatically become available to you after you complete the Nameless story quest. If you wish to romance Yennefer then this is the quest that you will have to complete to do so.
First thing you will have to do for this quest is to travel to the town of Larvik and speak with Yennefer here. She'll ask for your assistance in locating a Djinn that's said to be in the waters nearby. Now you will have to follow Yennefer to a boat at the dock, board it and sail out into the open seas with her.
If you're having trouble getting Yennefer to start sailing the boat, you have to sit down on the opposite end of the boat as her (as pictured below) - only then will she stop telling Geralt to hurry up and start sailing. The first area that Yennefer stops the boat will have some Drowners down near the bottom circling around some treasure chests - take them out and loot the chests before surfacing as they contain some crafting recipes.
The second area you come across with Yennefer will have an old war ship, inspect the body on the deck to advance the story forward. Check the southeastern portion of the rocks for more treasure chests and goodies. Depending on whether or not it is day out there may be Sirens in the air when you arrive at the next location. If there are, shoot them down with your Crossbow before proceeding.
When you stumble upon the underwater area that looks like a giant crater you'll have to collect the broken seal on the ground before returning to Yennefer. Back on the boat she will teleport you to a mountain top where there is another ruined ship waiting for you to inspect it.
You'll find a few crafting recipes throughout the ship as well as well as a bunch of loot, loot it all! To progress the story forward you'll have to go down to the second floor of the ship and inspect the shoes and streak of blood leading underneath a bookcase (pictured below). Have Geralt lift the bookcase up and you'll discover the body of Amos.
After a cutscene the Djinn will appear and Geralt will have to fight it. Use Quen as both Igni and Aard seem to do very little against it and roll around/slice away at it with your Silver Sword. Be careful not to roll off the side of the boat as the drop is an instant game over.
Djinn Weaknesses: Dimeritium Bomb, Elementa Oil
Once you defeat the Djinn you'll have another cutscene with Yennefer, during this cutscene depending on what you choose you'll be able to romance her. Choose to say something friendly to her and then tell her that you love her, you'll get another love scene for these options and the quest will conclude.