Don't you hate it how the ONLY way to survive long underwater is to make the potion? My suggestion is that maybe Minecraft should make armour, or at least ANYTHING else that will make you survive longer underwater. It's a big annoying how to always have to come up for air or else your hearts will lose faster than a ghast spitting fire charges at you. Do any of you agree?
We already have the Respiration enchantment for helmets, which slows down loss of air (1 minute of air at level 3), improves underwater vision, and slows down damage taken when drowning (40 seconds instead of 10 to drown without regeneration; on Peaceful you can actually survive forever as armor doesn't lose durability from damage sources not normally protected against).
Thus, I can't support this on grounds of being already in the game, and OP if it were as effective as water breathing (no air lost at all).
As for your suggestion, I don't really support it. We have a respiration enchantment, and getting water breathing potions is not that difficult, nor should you breeze through them in a couple of seconds. Water-breathing armor is unnecessary because you aren't really going to spend most of your life underwater (1.8 will probably change that), and the potions aren't difficult to make.
I know that, except something useful could be some armor actually made for surviving underwater, like scuba gear. And besides, as you say, the Respiration Enchantment is ONLY for Helmets. It would be nice if you can have a piece of footwear or pants that help you in the water somehow.
We have a respiration enchantment, and getting water breathing potions is not that difficult, nor should you breeze through them in a couple of seconds. Water-breathing armor is unnecessary because you aren't really going to spend most of your life underwater
Well, what if you want to make a "hidden base" in your world underwater? There should at least be something that helps you survive more underwater for the people who actually need it.
There is already respiration enchantment for breathing in water and there is depth strider enchantment for walking faster in water. You never actually suggested anything which is breaking the forum rules.