<p>Water clusters with distinct structures in confined nanochannels exhibit unusual transport phenomena such as fast permeation, yet how to functionalize nanochannels to regulate this process and enhance water transport remains elusive. Herein, we report a water transmembrane transport mechanism based on the dissociation of water clusters and, accordingly, develop a highly permeable nanofiltration membrane with polyhydroxy quaternized interface. The results, observed by in situ liquid time-of-flight secondary ion mass spectrometry in combination with molecular dynamics simulation, reveal that the functional groups (-OH and quaternary-N⁺) of membrane pore entrances can enthalpically/entropically favorably dissociate water clusters, i.e., (H₂O)₅H⁺, into smaller species, i.e., (H₂O)₃H⁺ with higher transport mobility through hydrogen bond interactions based hydration competition, which reduces water transmembrane transport energy barriers and broadens the cross-sectional area available to water molecules, thus improving the water permeance. The polyhydroxy quaternized membrane exhibits high permeability with a membrane flux of 43.01 L m⁻² h⁻¹ bar⁻¹, while maintaining favorable divalent salt retention performance and mechanical properties, which effectively improves the selectivity-permeability upper bound of the nanofiltration membrane. Our findings demonstrate an interface-functionalized strategy for regulating the water cluster structure, showing implications for various applications including nanofluidics, desalination, bio-medicine, and energy technology, etc.</p>