Hi Andy......depending on the vintage you are considering, most seagoing steamers used sea water cooled condensors...so the exhaust steam from the engine is fed to a condenser where it reverts to useable boiler feed water ......marine condenser shells are of a similar volume or dimension to the boiler shell itself
I addition to this, later vessels also supplemented this with sea water distillation ie., boiling and condensing the sea water to usable fresh water
Modern day cruise liners [with diesel engines] make all of their own fresh water when at sea........... Derek