Readers : As I have mentioned ; use what's available, only use no less than true distilled water.
My reasoning:
I recommend Peak Battery Water(now $2.99 a gallon @ PEP Boys): Its distilled, r/o'ed(was in the past) , de-mineraled and de-ionized for about $1.75 more than good commerical grade and a lot safer that supermarket distilled water. Very, very little of the stuff our water engineer friend mentioned to gunk/clog up the hand steamer.
As for my learning curve, it consists 25 years of "on & off" research on the subject of Steam Cleaning/Water related to Certification as the Specialist-In-Charge of Substance Abuse Detection for a the United States Courts Lab.
My work demanded perfection, nothing less was acceptable to the Courts. As the result, I learned by education and practice, all distilled waters are not alike. Much of the distilled water widely available fails purity standards for Labatory use. The only supermarket water my tech's used was to be mixed with pure bleach for cleaning purposes.
The first concern for all, safety ; heating tank explosions were common place from the 18th to the 20th Century when AC became the standard. God knows how many were mamed and died of scalding or explosion on the job. Remember Gentelmen, what we are holding in our hands , in its basic form ,is the same device just a little smaller... and of course safer. But how safe ? I want to keep the odds in our favor. Why? Murphy's Law is always present : Think Challenger.
Personally, I gave a lot of consideration to design factors before making the Perfection recommendation. However, one never knows if Unit A is so-much-more safer than Unit B since nearly all are manufactured over-seas where Murphy likes to sleez. As for me, I do safety checks on the steamers and did retire one (1) due to gunk. Since using Peak, the Perfection and SteamFast 227 have worked perfectly (to today).
The ultimate consideration ... Do what you believe is correct. All the best.