Minecraft wiki entry for Wither_Skeleton, associated text: Wither skeletons are immune to fire and the Wither effect. They still seek shade or water during daylight if they are in the Overworld although they do not burn in sunlight. Like most undead mobs, they sink in water but do not drown, are healed by Instant Damage and harmed by Instant Health effects, are immune to Regeneration and Poison effects and are not attacked by the wither.