How can I craft a new underwater helm?

Unfortunately, there is currently only one craftable breathing apparatus. It is the Water Filter. According to the wiki (thanks to Bazzz for the tip):

This recipe can be purchased from Stout Darkmind, the renown heart NPC at Blackblade Lake in Diessa Plateau for 126 [karma].

Unfortunately, it's pretty terrible.

The only other option for breathing apparatus replacement, as far as I know, is from a vendor. There are quite a few heart/karma vendors that offer upgraded breathing apparatuses.

You can now purchase ascended breather recipes from the "Aquabreather Recipes" section of BUY-4373 in the Mistlock Observatory and Sanctuary.

The recipes will appear in the helm section of your crafting discipline (tailor/leather/armour).

They are available in all stat types.