Since Furina loves to see a romance happen in real life, she decided to help you get into the underwater prison by getting Neuvillette to send you there with fake charges, so you can be there with Wriothesley who works as a warden most of the time now. But soon you will have to remember that since you enter in as a prisoner, you will be treated like one by the guards and prisoners while working and behaving without your personal items or vision.