Mobs don't take Damage from TNT that explodes in water

I wanted to farm music discs, I then set up a contraption with a skeleton that shots throught lava so the arrow is on fire so it ignites the TNT that then falls into a water pit with creepers in it. However I have placed like half a stack of TNT now and the creepers are still alive, I even trew 3 potions of harming 2 on the creepers but they are all still alive. I am very consfused.