Minecraft wiki entry for Drowned_farming, associated text: With a one-water-block trap, zombies just crowd into that one-block space, making it difficult to target the drowned zombies with your weapon. Allow the zombies to spread out as they drown by extending the trap to multiple blocks. In practice, a trap using a two-block water channel held up with two signs, with two landing blocks (or hoppers) below the channel, works sufficiently well. The illustration shows a design using a three-block water channel, three signs, and three hoppers.