The Breathing Reed is a tool found in underwater chests. When active, the tube will extend three blocks upward from the player. As long as the top of this tube is above the water level and within empty space, the player will be able to breathe. It also doubles the length of time the player's breath will last underwater (from 23 seconds to 46 seconds) and doubles the time it takes for their health to drain when out of breath. In order for it to take effect, the player must hold it.
When used in conjunction with a Mining Helmet or careful Torch placement, the Breathing Reed can be used to easily access underwater Chests. To create air pockets used for breathing, simply mine above an area of water. Note, however, that the tip of the reed must be in an air pocket to recharge your breath. The player's breath will not recharge if the reed is sticking into solid blocks (such as dirt). This only occurs if the character is underwater and is using the reed to breathe.
The Breathing Reed has one of the tallest drop-sprites in Terraria - approximately the height of a character.
The Fish Bowl will cause you to drown if you wear it in the equip slot, however, the breathing reed will prevent this.
The Breathing Reed stacks with the effects from a Diving Helmet, allowing for long periods of time underwater.
Its original tooltip was 'Cause not drowning is kinda nice.