Minecraft wiki entry for Drowned_farming, associated text: Using a glass-bottomed pool for the ceiling takes advantage of the fact that drowned spawn under water but not on transparent surfaces, so they end up spawning on the platform under the glass. The water attenuates the sky light by 2, and each block of space under the glass causes further attenuation. One needs at 13 blocks of space (including the glass) under the water for the light level to be zero on the platform to let drowned spawn during the daytime. Drowned also cannot spawn on bottom slabs, but using bottom slabs instead of glass for the bottom of the pool makes the farm inoperable. The bottom of the pool must be glass.