Marsh Wraith

The Marsh Wraiths are mobs that will attack any mob (including the player) that are in the water    in the Dead Marshes biome. You can not kill these mobs and they will shoot projectiles at you endlessly until you get out of the water. they despawn as soon as you are out of the water. They can also be seen in the water being inactive when you get too . They will constantly  an reappear in a new  in the water (the pool of water) you are looking in until you go or look away.