Romance Yennefer in this cute The Witcher 3 secondary quest.
Romance Yennefer - The Witcher 3: The Last Wish
If you want Yennefer to be Geralt's girlfriend when you reach the ending of The Witcher 3, this is the quest you need to do - enjoying the stuffed unicorn during The King is Dead - Long Live the King is not enough.
This quest becomes available after you complete the Act One: Skellige quest The Calm Before the Storm, but before you relocate to Kaer Morhen as part of Ugly Baby. That's a limited window of opportunity, so tackle it as soon as possible - straight after The Calm Before the Storm is ideal.
Meet Yennefer in Larvik (or accompany her there, if you prefer to take this option at the end of The Calm Before the Storm, and observe an altercation between Yennefer and some unruly locals in the local pub. Chat with her to discover her goal - a djinn (genie). Do not decline to help her now, or you'll close the romance chain forever.
Trot down to the local harbour and hop in the boat; you need to approach the bow (front part, opposite Yennefer), and interact to sit down, before she'll leave.
Once over the search zone, dive in - Yennefer casts a spell on you so you don't have to worry about breath, thankfully. Use your crossbow on any threats you encounter as you swim into a rocky tunnel and thence into the wreck. Use Witcher Senses to find a clan insignia - wrong wreck, alas. Return to Yennefer's boat to continue.
The second wreck isn't right, either, but you can loot a decent amount of treasure from it. The third dive spot is pretty interesting - swim down to discover a huge crater. This makes exploring much easier than when you have to twist and turn around obstacles, so it shouldn't take you long to nab what you're after - half of a magic seal. Feel free to grab any loot you find and look at any other objects of interest before returning to the boat.
Yennefer uses a portal to get you to the other half of the seal quickly - and the other half of the boat, in fact. There's plenty of stuff to grab in here, but your goal is a fallen bookcase on the lower deck. Investigate to reveal what's left of the djinn's former master. Yennefer speculates on what happened here: the consequences of mischievously literal wish interpretation. Search the body to find the other half of the seal.
Apply Elementa Oil and grab some Deimertium Bombs. Return to the top deck and watch as Yennefer summons the djinn. Unfortunately she can't bend it to her will immediately, so it's up to you to contain it - beat it up a bit.
When Yennefer brings the djinn under her control, the quest ends with cutscenes and dialogue. You have the option during this conversation to cement the romance or let it go. Get it right and you'll be rewarded with another love scene during a later quest.