Use ethanol, if you must use another solvent. Just be aware that it WILL burn when injected; and is also a great solvent for heroin, better than water, so use with care.
In the long term, ethanol will probably cause you grief too. But you only need to add a couple dozen to fifty units to forty units of water for good results.
I honestly don't see why heroin wouldn't dissolve in water, bro. It assuredly does. Perhaps these people have poor heroin? Or are heating the spoon and destroying their own dope? I mean, it makes no sense. Do they realize that in Kool-aid, the water is still the solvent? And worse, it's already solute-laden, possessing less available space for new solutes.
Honestly, cold water should be enough for any opiate.