Unfortunately, there is a bug in the current debian package of the achemso package for LaTeX. Every time you use \ref{key}, you will only get “1” instead of “Figure 1” although the latter output is the expected one. On top, the current gentoo package is up-to-date, hence it does not contain this bug, so if you happen to use both distributions you will have fun.
As a quick workaround, you may either perform a manual upgrade on debian or add the following lines to your document preamble:
\makeatletter
\def\p@figure{Figure~}
\makeatother
Please keep the tilde, as it will prevent line breaks between “Figure” and the subsequent number.