We explore the dynamics of low temperature interfacial water nanoclusters on Cu(111) by femtosecond-laser excitation, scanning tunneling microscopy and density functional theory. Laser illumination is used to induce single molecules to diffuse within water clusters and across the surface, breaking and reforming hydrogen bonds. A linear diffusion probability with laser fluence is observed up to 0.6 J/m2 and we suggest that diffusion is initiated by hot electron attachment and detachment processes. The density functional calculations shed light on the detailed molecular mechanism for water diffusion that is determined by the local structure of the water clusters.