In Titanic's case she had evaporators that turned seawater into fresh water suitable for the boilers - in fact distilled water, as any salts or other compounds or gases such as carbon dioxide and air can cause corrosion in the steam system.

The Titanic utilized a sophisticated technological solution for its time: the Weir Vertical Type Evaporator. This device functioned as an on-board desalination plant, using a tank of seawater and coiled heating tubes. Live steam from the ship's massive boilers was piped through these tubes, boiling the surrounding seawater. The resulting salt-free steam was then condensed into fresh water, which was primarily used to replenish the ship's boilers to prevent mineral scale buildup. For the passengers and crew, fresh water for drinking and washing was also stored in massive tanks located in the ship's double bottom. The evaporator was a critical backup; it could generate enough fresh water to sustain the ship's mechanical needs if the stored supply ran low during the Atlantic crossing. Interestingly, the highly concentrated salt water left over from the evaporation process, known as brine, was reused as a refrigerant in the ship's cold storage rooms because its high salinity meant it would not freeze at low temperatures.