Anionic water clusters have long been studied to infer properties of the bulk hydrated electron. In particular, the question of whether the excess electron is on the surface of the cluster or in the interior of the clusters has been the subject of much speculation. The successes of solid-state physics are built on exploiting the regularity of atomic arrangements in crystal. Describing the crystalline order of solids is relatively straightforward. Here we report the crystal structure of an anionic water cluster polymer [(H2O)18(OH)2]n2n− moiety that is stabilized by bis(2,2′-bipyridine) cupric chloride [Cu(bipy)2Cl]− host.