The Depth Galea is used to dive underwater. Wearing it is required to be able to go underwater, but once there the helmet can be removed. However, without it players will take 5 times 20 Drowning damage kicking in after four seconds of standing still. It is useful out of the depths, too: some monsters, like Water Elementals, use drowning damage attacks.
Part of the Depth Set. Functions as a lightweight, higher armored Helmet of the Deep.