Terraria (Wiki): Breathing Reed

The Breathing Reed is a melee weapon for underwater travel that pierces enemies.

The Breathing Reed is a tool that increases and allows breathing in water but will stop working for the duration of an attack once used. A Breathing Reed cannot autoswing. Its attack reaches four tiles in radius. At an optimal distance, the Breathing Reed can pierce two enemies. However, multiple bodies will be damaged should they all line up for the player.

When normally used, the Breathing Reed will not emit light, and attacks don’t go through walls. The underwater breath lasts for 46.67s when the breathing reed is held or for 3m and 6s if the diving gear is used. Additionally, the rate of health loss when drowning is slowed (1/2 or 1/8), respectively. 

The Breathing Reed needs to be held, which means it needs to be selected in the hotbar or attached to the cursor to work. The player won't drown if they hold it and have a Fish Bowl in their armor slot. When you hold this item, a reed appears above the player, and the speed at which their breath meter depletes and the speed at which they lose health while drowning is reduced by half. It will work even if the reed's tip is not above the water's surface. As soon as the reed's tip leaves the water but not inside a block, the breath meter refills. All Flask items will imbue their unique effects since the Breathing Reed is a melee weapon.

Combat-wise, the player's breath meter can refill underwater at a fixed depth, regardless of the Breathing Reed's size modifiers, which only apply during combat. It is effective against underwater creatures and waves of enemies with low health.

The Breathing Reed is best modified with Legendary for overall stats improvement.