The most popular medium-haul aircraft of the Airbus-320 or Boeing-737 families carry about 100-150 liters of water. Some of this water is used in the preparation of hot drinks for passengers.
Modern toilets on airplanes are vacuum, that is, they do not carry liquids with them, except for the secretions of passengers sucked from toilets.
Sinks are needed by for hygiene reasons - wash your hands.
Inflatable seats will not meet FAR durability standards. These requirements are mandatory for all developers.
The aircraft toilet is not water-based, it uses special disinfectant liquids. The number of toilets is also regulated by FAR.
Ovens are necessary to ensure your own comfort on board - when you are served lunch, you would hardly want a frozen product instead of a "hot" one, but for a long time