How can water catalyse a reaction between iodine and aluminium?
A naked aluminum surface (freshly sanded) in air immediately grows about 4 nm of adherent aluminum oxide that passivates the surface.  Iodine slightly dissolves in water and disproportionates

$$\ce{I2_{(s)} + H2O_{(l)} -&gt; HOI_{(aq)} + I^{-}_{(aq)} + H+_{(aq)}}$$

Iodine is strongly solubilized by forming triiodide.

$$\ce{I2 + I- -&gt; [I3]-}$$

The acids chew through the alumina surface barrier.  When bare metal and triiodide meet, everything takes off, also igniting the bare metal in air.

http://www2.ucdsb.on.ca/tiss/stretton/database/inorganic_thermo.htm
$$\begin{alignat}{2}
&amp;\ce{AlI3_{(s)}}\quad &amp;&amp;\Delta G_\text{f}^\circ = -300.8\ \mathrm{kJ/mol}\\ 
&amp;\ce{Al2O3_{(s)}}\quad &amp;&amp;\Delta G_\text{f}^\circ = -1582.4\ \mathrm{kJ/mol}\\
\end{alignat}$$
Absent water the iodine must diffuse through the intact alumina barrier, react with naked aluminum, then swell and disrupt the surface passivation from underneath.