Breathing Polyps are able to filter breathable air out of water. As such, any land-dwelling individual will be able to effectively breathe underwater if they insert one into his/her windpipe (April erroneously said she had one in her oesophagus).
The Maerum keep these polyps near to where they live. When they rescue/abduct sailors from the ocean surface, they bring them to special underwater breathing houses which have walls lined with these polyps, creating a suitable air pocket.
It is unclear whether the polyps occur naturally in these breathing houses, or if the Maerum farm them and place them in suitable venues. There is similar uncertainty about the breathing houses.
Obtained from the wall of the Landwalker's Bubble .