Minecraft wiki entry for Acquiring_a_conduit, associated text: To get started, the top two layers closest to the ocean surface yield 16 prismarine blocks and 4 sea lanterns. If you're lucky, the top of the ocean monument is close to the sea surface. You need only 16 prismarine blocks to construct a minimal activation frame to create an initial conduit. You can also use 180 blocks of sand to surround the top structure completely, allowing you to mine further down to get 12 more prismarine blocks of the top structure. Another way to get prismarine from an ocean monument is with an aqua affinity enchanted helmet and a potion of water breathing.