Ganache is a fat in water emulsion. Cream (and butter, in some recipes) provide the water, and butterfat (36% in US heavy cream). It's entirely possible to make a ganache using other liquids. Just water and chocoalte will make a ganache, if you get the proportions right (which is just a bit of math, so it's easy enough), but it won't have the same mouth feel and texture that cream ganache does. Adding some fat, in the form of oil, will improve things (and give you an interesting flavor source, too, if you're using something like nut oil with a strong flavor). Better is cocoa butter, which has less flavor, but more suitable melitng point. If you're willing to do shell molding, I'd also consider coconut oil, which when mixed with cocoa butter (from the chocolate, you don't need to add any) will have a eutectic effect, meaning the melting point is lower than you'd expect, which will let you make a sort of meltaway truffle. You can use flavored water (tea, infused with herbs, maybe even coffee (haven't tried that)).