Minecraft wiki entry for Acquiring_a_conduit, associated text: Once you have the 5×5 ring, put the conduit in the middle to activate it. Once it activates, it grants the Conduit Power effect, which allows the player to breathe indefinitely, see better, and mine faster underwater. Then you can breathe underwater inside a radius that covers the entire monument, allowing you to get more prismarine to move your conduit elsewhere later.