Johnny Deepend mainly attacks by throwing water polo balls, even when Luigi first enters his room. Since he immediately dives out of reach if Luigi activates the Poltergust G-00, Luigi has to send Gooigi through the fences around the pool to reach a wheel that will drain the water out of the pool. If Gooigi is active, Johnny Deepend will attempt to dissolve him by spitting water at him; while Gooigi can take cover behind the pillars around the room, he'll be completely exposed while he drains the water. As such, Luigi can temporarily knock Johnny out by hitting him with a water polo ball, giving Gooigi time to get to the wheel and turn it with his vacuum.
Once the drain is opened, Luigi or Gooigi can descend into the now-drained pool to confront Johnny Deepend, who is now stuck in the open drain. At this point, the ghost's only attack is to slam the ground with his fists; although this deals significant damage, it has limited range. Luigi (or Gooigi) needs to stun Johnny with another water polo ball, allowing them to remove his swimming goggles with their vacuum; afterwards, he is finally vulnerable to being stunned and captured.
When his HP is depleted, Johnny makes some poses before pinching his nose and being sucked into the Poltergust, allowing Luigi to claim the elevator button. However, much like with Director Morty, he'll have to deal with Polterkitty stealing the button when he goes to put it into the elevator.
His name is a pun on famous actor Johnny Depp and the "deep end" of a swimming pool. He is a muscular black-haired ghost wearing a green leotard, a striped swimming cap, and a pair of swimming goggles.
Johnny Deepend has the second-lowest HP of any boss ghost in Luigi's Mansion 3, with Morty having the lowest HP.