Minecraft wiki entry for Drowned_farming, associated text: The farm becomes operational as soon as the glass-bottomed pool is filled and you descend to the killing platform. Whack at the drowned legs with your sword (or your fist), and start collecting loot and experience. Fish also spawn in this farm and mostly swim upstream avoiding the fall, but once in a while they fall through and die. This does not affect drowned spawning, but expect to see an occasional raw cod or salmon appear in your chest.