Strangely, my answer would have been "weakly tetrahedral hydrogen bonding" when water has a regular tetrahedral lattice, it's ice. It's when there are disruptions to the lattice that things are more interesting.
"When water freezes into ordinary ice, which is the kind that makes the ice cubes that float in our highballs, this happens at what we would call zero degrees centigrade, at atmospheric pressure. When water freezes into ice it creates a very open structure. That form of ice comprises arrays of six membered rings that are stacked on top of one another to make channels and most of that ice is actually empty space."