I just use a steaming pitcher full of cold fresh water, give it around 15-18 seconds with the steam wand and am good to go.
This is my preferred method. If you have a commercial grade machine or semi-commercial [anything with a large capacity boiler (HX or double boiler) ] this is your best option.
If you have something with a small boiler such as a silvia or a small lever machine, you should use an electric hot water kettle or something similar for the water.
I could only imagine how miserable it would be to steam a large cup of water with the silvia boiler only to have to wait for it to cool down and stabilize to make espresso