Summoned a Wither using /summon minecraft:wither underwater next to a fully powered conduit. It didn't finish regenerating, but instead, managed to take damage before it was finished. It then kept having exploding cube auras around it appear every so often as long as it was underwater These auras appeared to heal the Wither. It had to be lured above the water in order to deal effective damage, and even then, it still healed at an odd rate. In addition, the Wither somehow managed to break a couple of blocks underwater, despite the blocks being unable to break by explosions when submerged in water.