You can also find Armor pieces with enchants for non-stop Waterbreathing, same as Argonians. Some helms I've seen don't make sense. I've once found a Scaled Helmet of Waterbreathing.. like wait how? There's no closing for the face.. how can you use that to breathe underwater? Gonna breathe through the top of your head? LMAO.
Helmets like Ebony/Imperial Legion Helmet/Dwarven, etc. make to be more reasonable lol.
Magic.

Think of the Waterbreathing enchantment as an invisible magical bubble over your head. The item is only there to bind the magic to you, so you might as well wear a circlet or helmet of Waterbreathing, it comes down to the same thing.