Not sure if you are only asking about very old steam ships. More modern designs, as used by various naval forces as well as commercial operators, still use stea

Not sure if you are only asking about very old steam ships. More modern designs, as used by various naval forces as well as commercial operators, still use steam to convert thermal energy to mechanical energy. This is true whether they burn crude oil or even have nuclear reactors. They have methods to "make water" by using some of their energy to run some kind of desalination plant. Traditionally, this was a distillation process. Newer ships might have reverse osmosis systems. I don't know which are preferred in military applications. Reverse osmosis is more energy efficient, but I can imagine that distillation may have more reliability under adverse conditions. I could imagine having both for efficiency with fault tolerance... They would still also have a reservoir to store water for later use. That way, all the available thermal power can be directed towards locomotion when necessary. They can make water periodically, when power demands allow it and in a manner that would be most efficient for the equipment. The desalination may not be perfect either. As I understand it, multiple US Navy ships inadvertently exposed their crews to Agent Orange during the Vietnam war through this process. They were not directly exposed to the chemical like victims on the land. Instead, the run-off from land went into river deltas and contaminated water was drawn in by nearby ships. The chemicals made it through the desalination process and into drinking water supplies.