Minecraft wiki entry for Man-made_lake, associated text: 3) If it's an under water current, it means there's a hole in the wall or a stray block is blocking the water above this point. If it's a hole, fill it with either a spring of water or a block according to your preferences/plans. if it's a stray block, destroy it to allow the passage of water or if you want to keep a block there, fill the hole under it with water.