What is the structure of small clusters of water molecules?Structures of Cage, Prism, and Book Isomers of Water Hexamer from Broadband Rotational Spectroscopy. The associated Perspective by Saykally and Wales is particularly helpful.
In particular, for six molecules there are three alternative structures which are very close in energy, denoted the prism, cage, and book, in the figure above.
Quantum chemistry calculations give different energetic orderings depending on the level of theory used. Furthermore, the zero point motion of the atoms is important.
The high resolution spectroscopy in the paper suggests that the cage structure is the most stable, but only by an energy of about 1 kJ/mol (~10 meV). Contrary to what one may expect, not all the bond lengths are equal.