Field ionization of water-ice adsorbed onto a platinum field emitter tip of radius 350 Angstrom was studied as a function of temperature over the range of temperature 80-145 K and of water layer thickness 100-3000 Angstrom. The water adlayer was grown under field-free conditions by exposure to water vapor in ultrahigh vacuum. Field ionization was probed by ramped field desorption (RFD), in which desorption of ionic species (hydrated protons) is measured while increasing the applied electric field linearly in time. The dependence of the field required for onset of ionization as a function temperature and thickness is presented and discussed. In the limit of thin water layers, the onset field of ionization decreased from 0.6 to 0.3 V/Angstrom with temperature increasing from 80 to 145 K. An activation barrier of 0.75 eV for ionization of water to produce hydrated protons and hydroxide ions was estimated from the temperature dependence of the onset field. The onset field increased with water layer thickness, and a break point in the slope of onset field versus thickness was interpreted as a transition in the ionization location from the water-vacuum interface to the tip-water interface. The relevance of these experiments in simulating electrode-electrolyte interfaces is discussed.