If you are going to place an electric heating element in a tank to make an electric water heater, then you have two proposed layouts: vertical and horizontal layouts.
In fact, the vertical layout of the heating element is going to be affected by changing water level due to consumption leading to overheating of some portion of the heating element which may result in burn and failure of the heating element. Also, the vertical layout in most cases is just wasting energy and inefficient. Furthermore, in the vertical layout, the natural buoyancy potential is decreasing when moving up along the heating element.
On the contrary, in the horizontal layout, the heating element is always immersed (fully covered) under water and so it is protected from overheating and efficient.