State-of-the-art quantum simulations on a full-dimensional ab initio potential energy surface are used to characterize the properties of the water hexamer. The relative populations of the different isomers are determined over a wide range of temperatures. While the prism isomer is identified as the global minimum-energy structure, the quantum simulations, which explicitly include zero-point energy and quantum thermal motion, predict that both the cage and prism isomers are present at low temperature down to almost 0 K. This is largely consistent with the available experimental data and, in particular, with very recent measurements of broadband rotational spectra of the water hexamer recorded in supersonic expansions.