Nanostructures can be patterned with focused electron or ion beams in thin, stable, conformal films of water ice grown on silicon. We use these patterns to reliably fabricate sub-20 nm wide metal lines and exceptionally well-defined, sub-10 nanometer beam-induced chemical surface transformations. We argue more generally that solid-phase condensed gases of low sublimation energy are ideal materials for nanoscale patterning, and water, quite remarkably, may be among the most useful.