This is not allowed where I'm at, and pretty much everything is allowed here. We don't have any building Code outside of city limits, but if you want to have "city" water, then you have to do it their way.
It is written in stone that water and electricity must go in separate trenches with at least a foot of virgin soil between them. If you cross, then water MUST be on top of power.
The reasoning is that if you ever have a water line break, and they do all the time, you do not want to be digging in a hole full of water and risk hitting the power line. If the water line is below the power line, it is very easy to hit the power line while getting down to the water line.
I know from personal experience that if you decide to set a fence post, or dig into the ground for any reason, you will hit a power line eventually. In the last couple of years, I've cut two lines going through my yard.
Even if you know exactly where those lines are, there will be a time when you are not able to do the work yourself. Then it will be a friend, family member or contractor that is digging up that broken water line, and risking their life if the power line is above the water line.
Putting water and electricity in the same trench should never be done.