Technically you only need either a qualified plumber or electrician to install your electric water heater. You don’t need both to do it. A plumber has the expertise to install the product without any interference or glitches in your electrical circuit board.
To avoid any problems installation should be done by a qualified plumber or electrician that knows exactly where all the nuts and bolts go. Your warranty can be waved and considered void if the installation was not done properly as per the manufacturer's instructions. It's simply not worth the risk to install yourself or getting a layperson to assist you.