[en] Highlights: • Interfacial water forms cubic-ice crystallites with a {1 1 0}-preferred orientation. • Four stages of ultrafast structural dynamics of interfacial water are presented. • A nonequilibrium isotropic phase transformation of interfacial ice is identified. • Cooperativity of a hydrogen-bonded network inhibits sublimation during large motions. Using time-averaged and ultrafast electron diffraction, structures and ultrafast dynamics of interfacial water assemblies on smooth hydrophobic surfaces are reported. The lack of hydrophilic interaction and topographical template effect from the support surface leads to the formation of small, mostly randomly-oriented, ice crystallites with the cubic structure. Dynamically, following the substrate photoexcitation, interfacial water assemblies undergo four stages of changes—ultrafast melting, nonequilibrium isotropic phase transformation, annealing, and restructuring—which are closely correlated with the substrate dynamics. The connectivity and cooperative nature of the hydrogen-bonded network is considered crucial for water assemblies to withstand large structural motions without sublimation on ultrashort times.