Do yourself a favor and use the water the instructions say. It will make your iron last much longer and perform better. I know many folks here are not fans but I am a Rowenta girl. I've had 2. One said to use distilled, the other tap. It does make a difference in the lifespan and performance.
Distilled water is not that expensive and can easily picked up in the grocery store on your regular shopping trips. That's what I do at least.