If you are planning to boil water at low pressure, it will happen at low temperature, which means you need an even lower temperature to condense the spent steam back to water. An efficient heat engine will operate across a significant temperature difference - the bigger the better. Dissipating heat in space can only be done by radiation; the higher the temperature, the higher the rate for a given surface area. That means a practical steam engine would work more efficiently using a high temperature (therefore high pressure) boiler and a high temperature radiator to condense the spent steam (also necessarily operating at significant pressure). Ambient pressure (or lack of it) is irrelevant.

Steam engines on Earth are efficient because they boil water at high temperature and almost always have access to an effective heat sink in the form of a large river (for stationary power plants) or the body of water on which they operate (ships) and can condense spent steam at relatively low temperatures because of the efficacy of the available heat sink.