The fresh water used for drinking was filled from the side of the hull from the wharf or a water tender. It was stored in the six tanks abeam the electric generating compartment. This was specifically for consumption. Washing water was stowed in the double bottom in specific tanks. The fresh drinking water and washing water was pumped from below to the elevated boat deck tanks where it was stowed to allow it to feed the main by gravity (this is like a village water tower). Though salt water could be distilled on Titanic, it was not cost effective, and done in emergencies only. If the distilled water was used for consumption, it would have to be passed through an aeration filter to bring it to life. These devices were on Titanic and liners of the era, but used as a back up. Otherwise the distillers etc., were used for making boiler feed water which was also loaded from a tender, and only manufactured if needed.
The fresh water that came out of the first class lavatory basins and private bath shower heads, Smiths shower etc. was of this washing water quality (if you will) and was not intended to be consumed. Water that was intended to be consumed was brought to the room by a steward, and was supplied to the galleys and pantries. The fresh drinking water passed through a Pasteur filter at some point in its travel to the tap. Whether this was done through a filter in the engine room, or filters at the service points (most likely) I am unsure about. There were water coolers on this ship in areas also. When one sees a glass flask next to a fold down or marble lavatory basin with drinking glasses below it etc., that drinking water did not come from the sink (or it should not have). It was brought to the room by a steward/ess.
The hot salt water used for baths was of course pumped from the main condenser discharge where it was already pre-heated, and sent to storage tanks were it was maintained to the prescribed temperature. The cold salt water from bathing and the pool came right from the ocean.
Hot washing water was maintained in 'tween deck tanks and rose up to the underside of the service giving immediate hot water upon opening the tap. The overflow from these "rising mains" returned back into the system in most cases.
Water used for flushing the sanitary system, toilets and urinals, as well as those systems used for wash deck purposes and fire mains came right form the ocean, but unlike the 1911 Olympic and many other ships, was not stowed in a large tank on the Boat Deck. The water pressure was maintained in the mains by direct pumping. Olympic was converted to this system, removing the sanitary water tank in 1912-13 to make room for the engineer's smoke room, while Titanic and Britannic were built with this feature from the beginning.
The swimming pool, or bath, whatever one wants to call it, was actually a large bath tub. It had both hot and cold salt water mains going to it from the ship's corresponding service mains. It should be noted that the drain and overflow from the pool went into the No. 5 Stb wing tank, which seems to have been the receptacle from other overflows also.
Bruce Beveridge
Sorry for any spelling mistakes - in a hurry as usual