'Boiling' isn't a noun.

true, but you'd be wrong to assume that Germans only compound words if both parts are nouns, e.g. "Gehweg" (walk way) and "Springseil" (jump rope) use the base of a verb. We do actually have "Kochwasser" ("kochen" means "to cook", "kochend" means "boiling") but that's not boiling water ("kochendes Wasser") but for water used for cooking.