Turtle shell pieces >> helm only; interesting limit.
Given the superior nature of Respiration3 to 10 seconds of waterbeathing (as well as the 3 vs 2 armor value), I think turtle shells will probably be a (largely) early game bit of kit…
(especially if the other standard helmet enchantments [Aqua Affinity, Protection X 4 and Unbreaking3] can not also be applied).
[I raise the issue because of the item name "Turtle Shell" which suggests it [i]may[/i] not be considered a helm for purposes of recieving enchantments.]
EDIT: looking at some of the snapshot vids and some other posts, it may be that the waterbreathing lasts 10 seconds after the headgear is removed — which would make it much more useful… (and a real competiitor for the current uber helm)