In what ways can you acquire Aquatic Helms?

It is possible to craft a Water Filter with the Armorsmith crafting discipline, but that recipe must be learned from a Karma vendor (Stout Darkmind) too, and it is only level 20 and only available to heavy armor classes.

They can also be attained randomly by combining other pieces of armor at the Mystic Forge.

Rare, level 80 aquatic helms can also drop from defeating Tequatl the Sunless. These were added in the September 17th, 2013 patch, and can be bought on the trading post. These are the most powerful aquatic helms, but are only available in Soldier's stats (Power / Toughness / Vitality).