Cap of Water Breathing

Wondrous Item, Uncommon

While wearing this cap underwater, you can take a

When you take the Magic action, you cast a spell that has a casting time of an action or use a feature or magic item that requires a Magic action to be activated.

If you cast a spell that has a casting time of 1 minute or longer, you must take the Magic action on each turn of that casting, and you must maintain Concentration while you do so. If your Concentration is broken, the spell fails, but you don't expend a spell slot. See also Concentration.

action to create a bubble of air around your head. This bubble allows you to breathe normally underwater. This bubble stays with you until the cap is removed or you are no longer underwater.

Source: Dungeon Master's Guide, p. 242