The bug
Drowned mobs are not included in the advancements "Monster Hunter" (minecraft:adventure/kill_a_mob) and "Monsters Hunted" (minecraft:adventure/kill_all_mobs).
MC-127252
Killing the Drowned mob doesn't count towards the Monster Hunter/Monsters Hunted advancements
Resolved
Fixed
12
Confirmed
advancement, drowned
Minecraft 18w11a, Minecraft 18w14a, Minecraft 18w14b, Minecraft 18w15a, Minecraft 18w16a, Minecraft 18w19b
Minecraft 18w20a
Retrieved
Still in 18w14a