Water Breathing is an Adept-level Alteration spell. Casting it allows the user to breathe underwater for 60 seconds. It cannot be cast while underwater or swimming. Casting it repeatedly while standing in water is an effective way to level the Alteration skill. Casting Candlelight ten seconds before casting water breathing is good indicator of when you need to surface to avoid drowning.
Potions with Water Breathing may be brewed at an alchemy lab. When consumed, they temporarily allow the user to breathe underwater. The strength of the effect is determined by the Alchemy skill of the potion's creator.
Helmets, amulets, and rings may be enchanted with Water Breathing at an Arcane Enchanter. Apparel enchanted with this effect allows unlimited breathing underwater as long as they remain equipped. The enchantment's effect is not quantified, so it does not vary with the size of the Soul Gem used: i.e., a Petty Soul Gem grants the same effect as a Grand one. The Item ID for this effect is: 00092A48
Curiously, even when wearing an item enchanted with Water Breathing, the player may still be heard gasping for air upon breaking the water's surface.
Equipment enchanted with Water Breathing may be randomly obtained as dungeon loot or from merchants, albeit extremely rarely. One way to find a piece with the enchantment is to utilize a skeleton at Cradlecrush Rock that always holds a random piece of enchanted headgear. Simply save somewhere far from Cradlecrush Rock, travel to the area, and check the skeleton. Repeat until an item with Water Breathing is found.