Pictures are worth a thousand words, but...it's dark outside and I'm too lazy to draw a diagram. So, let me try to paint a picture with my words.
I live in a single story house with no basement. Looking at my house from the street, power runs to the right side of the house, and water runs to the left side. My kitchen is in the front on my house on the right, and the electrical box is on the outside of one of my kitchen walls. My garage is on the left side in the front, and the water heater is located against the left wall of the house but inside the garage. So, the water heater and electrical box are about as far apart (relative to the width of my house) as they could be. If you were able to walk from the electrical box to the water heater, you'd walk through my kitchen, a hallway (from the front door), and the garage. But, you can't go that easily in the attic because the front face of my house isn't a straight wall, nor is the roof line. So, running anything through the attic isn't a straight shot. Running anything underground in front would require going under the driveway.
I would want the water heater inside of the house, or at least inside of an environmentally sealed and locked box outside, because (a) the controls don't seem to require any password/code, and (b) the metal case isn't environmentally sealed, so all of the electrical lines could get wet outside. I'd also prefer it inside because I'd like to be able to adjust the setting without going outside. I wouldn't want it inside of the kitchen (near the electrical box), but instead would like it either in the garage where the other one is or maybe (maybe) in the laundry room. So, I'd have to run electrical lines somewhere, and if I put it out in the garage, then I don't have to also run water lines.
I'm guessing if you were here, you would know the easiest/cheapest way to do it. However, I also don't think it's as straightforward as you were thinking. There's no incredibly easy way to get water or electricity from one side of my house to the other in a permanent code-compliant way. I'm pretty sure it would have to go through the attic.