From the Super Mario Wiki
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The Rooftop Pool is a room inside the Haunted Towers in Luigi's Mansion: Dark Moon. The Rooftop Pool can be accessed only by inspecting the reclining chair inside the West Bathroom. This large room, as the name implies, has a pool in the middle with a diving board. There are frog statues on the three corners of the pool; the other corner has stairs to get in and out the pool. There are many vines creeping along the windows and floor of this room as well as several hanging plants.
In Pool Party, Luigi needs to make his way to the Rooftop Pool to find the Crypt Key that is stolen by the Boos from the Crypt. When Luigi gets to the pool, he should shine his Dark-Light Device on the top left corner of the pool. This reveals a frog statue, and after all the Spirit Balls are vacuumed, it spits out the Crypt Key. When Luigi approaches, a Sneaker steals it. Luigi needs to fight four Sneakers after this encounter. After he defeats them, the Polterpup steals the key from Luigi and jump out the window, ending the mission.
In Doggone Key, it cannot be entered due to the chair still being up.