The adsorption kinetics of water on Ru(001) was simulated using molecular dynamics (MD) and equilibrium-model approach. The results nicely reproduce observations from STM imaging, work function change and IR measurements. The agreement with experimental results is based on the formation of stable clusters already at very low surface coverage and temperature. Tetramers are predicted to be relatively stable compared to smaller and larger clusters. The dipole moment per water molecule continuously decreases from 2.2D for the monomer down to 1.1D for pentamer and larger clusters. Dimers diffuse faster than monomers or larger clusters, in agreement with STM measurements. A unique mechanism for dimers diffusion is proposed. Photo-dissociation of H2O/Ru(001) with 193nm photons support recent DFT calculations that claim thermal decomposition.