I use tap water (we have our own well and do not have to soften our water)
From the Rowenta website:
WHAT WATER MAY BE USED ?
Your iron has been designed to operate using untreated tap water. If in doubt, check with your local water authority. However, if your water is very hard, you can mix 50% untreated tap water with 50% distilled or demineralised water.
IMPORTANT Heat concentrates the elements contained in water during evaporation. The types of water listed below contain organic waste or mineral elements that can cause spitting, brown staining or premature wear of the appliance: water from tumble dryers, scented or softened water, water from refrigerators, batteries or air conditioners, pure distilled or demineralised water or rain water should not be used in your iron. Also do not use boiling, or bottled mineral water.
***Also, when you're finished ironing be sure to empty the tank. Don't allow any leftover water to sit in there.