Other than the one Deadric mask, that is (one of them grants you water breathing but I hate wearing it) ? In Oblivion, you could buy or acquire some form of jewelry that bestowed the water breathing ability. I've continued to look in Skyrim, but have yet to find one or learn how to craft one. Anyone know?
I just got that mask, I think it's pretty awesome actually. It has water breathing, 20% better prices, and +20 carry weight. The Volkun mask I think it's called.
But other than that and waterbreathing on other helmets in the shop (but rarely, in my experience) I've never seen it on an amulet yet. I don't think you can enchant an amulet with waterbreathing either, so maybe it's not possible in this game.
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I found a leather helmet with water breathing on it in some random loot as I was just exploring. Disenchanted and made a ring with it.
I went back there on a another character and found a completely different item. Loot is random like that I guess. Just like how I can never find an item with soul steal on it, unless it is on a character that I dont need it on.