With the water supply to the house turned off, then any potential leak would be limited at maximum to the 40 gallons....not pretty, but probably not a disaster if it did happen. I do recommend turning off the main water any time a unit is vacant more than a day or so. I do that at home.
With an electric, of course turn off the juice...and very important: before turning on the juice again, run a hot tap for few minutes to ensure the tank is full and any air is out.
I would let your installer provide the necessary install materials.