Spatial separation of water dimers from water monomers and larger water clusters through the electric deflector is presented. A beam of water dimers with 93% purity and a rotational temperature of 1.5 K was obtained. Following strong-field ionization using a 35 fs laser pulse with a wavelength centered around 800 nm and a peak intensity of 1014 W/cm2, we observed proton transfer and 46% of ionized water dimers broke apart into hydronium ions H3O+ and neutral OH.