Minecraft wiki entry for Drowned_farming, associated text: The walls around the spawning platform should be tall enough to provide sufficient darkness on the spawning floor. In Bedrock Edition, it should be 8 or 9 blocks tall to guarantee sufficient darkness on the floor after the transparent roof is in place. In Java Edition, you will need the wall to be at least 9 blocks high to accommodate a layer of signs and several layers of water (17 layers of water guarantees total darkness on the spawn surface). Place slabs on the top of the wall to prevent mobs from spawning on it.