Not exactly sure why you want to sidestep the artificial sweeteners (which even if benign chemically probably affect taste->brain communications), the phosphoric acid (which may affect mineral balance in the body), and/or the caramel coloring (which is linked to cancer).
But regarding your question, I don't think diet sodas contribute to holding onto fat (don't see a mechanism aside from causing increased eating as a result of the sweet taste and no calories). Nor do they likely contribute to water retention since given the caffeine and the very low levels of saltsodium, if anything, they are essentially diuretic in nature.
