Minecraft wiki entry for Guardian, associated text: Guardians spawn naturally in ocean monuments. They require water to spawn. Specifically, they can spawn only if the spawning block and the block below the spawning block are both water (regardless of stationary water or flowing water). In Java Edition guardians spawn less often in ocean open to the sky than they do in covered areas (such as inside or underneath the monument). Specifically, spawning fails 95% of the time if the spawning water block is below sea level (Y=63 by default), all blocks between the spawning water block and sea level are liquid or fully transparent, and the block at sea level has a view of the sky.