Board Thread:Skyrim/@comment-14.203.34.26-20141116000140/@comment-88.115.105.16-20141116103009

I have full perks in alchemy and enchanting, and it provides waterbreathing potions up to 191 seconds. But as said above, the best option is to find the items with the enchantment, they are just somewhat rare.

In the Inner sanctum one of the frozen falmers has one circlet of waterbreathing, and if I remember correctly, also the Ebony Warrior has that in his helmet. Other than that, there are vendors and other random locations (check the waterbreathing enchantment from wiki, it should provide you some locations where to find those) :)