The Boilerworks is the second basement floor as well as the bottom-most one of The Last Resort in Luigi's Mansion 3. It is accessible after capturing Ug at the Unnatural History Museum and obtaining this floor's elevator button from the Toad he was guarding. Shortly after arriving here, Luigi finds the mechanic ghost Clem sleeping. Luigi accidentally steps on a rubber duck, waking him up. Clem turns the valve on a pipe, causing it to squirt water in Luigi's face, at which Clem laughs before leaving. Next to the wheel is a fake wall that can be removed to reveal a pipe Gooigi can go through, which leads to a key needed to unlock the door to the next room. In the Waterworks, Luigi finds a Goob with a bucket on his head getting hit by Goob with a monkey wrench. More Goobs, as well as Mini Goobs, appear after this, and Luigi cannot continue until they are gone. After beating them, Luigi needs to turn three valves to alter the flow of water so Gooigi can unlock the door that leads into the Reservoir.
When Luigi reaches the top of the Reservoir, Clem pulls a lever that floods the Boilerworks, then scares him into the water. Polterpup saves him from drowning, and Professor E. Gadd calls, telling Luigi that the Poltergust G-00 is waterproof and that he will have to get to the Drainage Control area so he can drain the water. Luigi must ride inflatable rafts through the flooded sewers while avoiding mines, with Gooigi crawling through pipes and opening gates for him. Luigi must also open pipes for Gooigi. After Gooigi opens the final gate, Goobs with shark fins will attempt to pop Luigi's raft. At the Drainage Control area, Luigi gets attacked by a Slinker and some Mini Hammers. Once they're gone, Luigi needs to turn some valves on some pipes so Gooigi can go through and pull the drainage lever.
With the water drained, Luigi can now go back through the sewers and return to the Reservoir. On his way back, he finds the shark-finned Goobs flopping around like fish out of water. Their goggles protect them from the Strobulb, so Luigi must burst next to them to make them vulnerable. After returning to the Reservoir, Luigi can climb up to the Main Tank room, where he once again finds Clem sleeping. Just like last time, Luigi wakes him up by stepping on a rubber duck. Clem fills the tank with water, commencing a battle on inflatable rafts. When defeated, he drops the elevator button that leads to the tenth floor, the Tomb Suites.
Later, after Luigi finds himself unable to progress at The Spectral Catch, E. Gadd informs Luigi that one of the Toads has gone missing while attempting to retrieve one of his inventions at the Boilerworks, forcing Luigi to return here and escort Toad until they can make it back to the elevator and return to the lab with the invention.