There's been gilled creatures in the market for sale, of course, but they aren't something Shmi and Anakin could have afforded so he's never seen much detail of one. And he's not imaging how gills work properly, but in a way that makes sense to how he understands breathing to work.

"Are there fun ways to die?" He's not meaning to sass - death is too much a part of a slave's life after all, and he's seen more of the brutal reality of it than someone his age should have.