The molecular lattice

Apart from the ionic and the metalic lattices, there are molecular lattices.
As the word says: this is a lattice built up of molecules.
In such a molecular lattice, the molecules hold each other in a lattice with there mutual attraction forces. These forces can be: vanderwaals, polar or H-bridge.
The vanderwaals forces depend on the mass of the molecule. The bigger and heavier a molecule, the stronger the molecular lattice.

That's how sugar molecules behave in a lattice to form cristals. The forces that hold those molecules toghether are reasonable big, because the sugar moleculas are rather heavy (and also polar).
So there are also polar forces working here.

Water molecules are much lighter, but very strong polar. They remain toghether, not so much because of the vanderwaals forces, but mor because of the polar forces.
The final effect is that they only form a lattice at 0oC (freezing point)