Tap water would be fine, PROVIDED it has a low hardness content. When heated, hard waters precipitate calcium and magnesium carbonate, which is scale very hard to remove. In this case, soft water would be advisable.
Deionized water, on the other hand, would be perfect, but is very expensive. Unless you have it readily available, replace it with soft water.
Just to give you a preliminary idea, do you know what's the hardness content of your tap water?
Giovanni S. Crisi
Sao Paulo - Brazil