I have this realistic version of Nintendo's Kirby.
It can suck in air (atmosphere on the world I'm putting it on is dense) and expand by using its bellow like lungs to rapidly pump the air into its gap layer beneath the stretchy skin (connected to the main body by flexible scaffolding; the face and limbs do not have this gap). It can then vent out the air through two port holes on the back to get a boost. This is used to defend against predators: to make itself appear larger; to escape by becoming airborne and jetting away. For sucking in prey, they have a filter to keep things from entering the lungs as they suck in.
But it is also semi aquatic. Meaning it must also be able to do this underwater. So could a lung-like organ be capable of extracting oxygen from both air and water? What characteristics must it have?
Yes through skin breathing would work, but I should have specified: due to how the bellow lungs work to inflate the creature or suck in prey, and the fact that this needs to also work under water, means that these lungs will have to push water as well, and I'm not sure how well lungs could handle that and what changes would need to be made to make it capable.