This is neither a problem with QuickLaTeX nor with chemmacros per se. chemmacros uses tikz, a fact which must be told to quicklatex:
[latex][+preamble]\usepackage{chemmacros}[/preamble]
\begin{tikzpicture}\end{tikzpicture}
\ch{2 H2O -> H3O^{+} + HO^{-}}
[/latex]
