A hybrid water storage system for an aircraft, the hybrid water storage system comprising: a container defining a chamber; a first variable volume fluid vessel disposed in the chamber, wherein the first variable volume fluid vessel is configured to store potable water; and a second variable volume fluid vessel also disposed in the chamber, wherein the second variable volume fluid vessel is configured to store gray water, the second variable volume fluid vessel being defined between an external surface of the first variable volume fluid vessel and an internal surface of the container; a water-use assembly extending between and fluidly interconnecting the first variable volume fluid vessel and the second variable volume fluid vessel, wherein potable water is supplied to the first variable volume fluid vessel via a fill port and is directed to the water-use assembly via a delivery port, and wherein gray water is returned to the second variable volume fluid vessel via a return port and is discharged from the second variable volume fluid vessel via a discharge port; wherein the water-use assembly comprises a gray water sump and a gray water pump, wherein the gray water sump is configured to receive the gray water from an aircraft appliance and the gray water pump is configured to pump the gray water from the gray water sump to the second variable volume fluid vessel; a controller configured to actively control a water transfer between the first variable volume fluid vessel and the second variable volume fluid vessel via the water-use assembly; and a gray water overflow container coupled in overflow receiving communication with the second variable volume fluid vessel. a container defining a chamber; a first variable volume fluid vessel disposed in the chamber, wherein the first variable volume fluid vessel is configured to store potable water; and a second variable volume fluid vessel also disposed in the chamber, wherein the second variable volume fluid vessel is configured to store gray water, the second variable volume fluid vessel being defined between an external surface of the first variable volume fluid vessel and an internal surface of the container; a water-use assembly extending between and fluidly interconnecting the first variable volume fluid vessel and the second variable volume fluid vessel, wherein potable water is supplied to the first variable volume fluid vessel via a fill port and is directed to the water-use assembly via a delivery port, and wherein gray water is returned to the second variable volume fluid vessel via a return port and is discharged from the second variable volume fluid vessel via a discharge port; wherein the water-use assembly comprises a gray water sump and a gray water pump, wherein the gray water sump is configured to receive the gray water from an aircraft appliance and the gray water pump is configured to pump the gray water from the gray water sump to the second variable volume fluid vessel; a controller configured to actively control a water transfer between the first variable volume fluid vessel and the second variable volume fluid vessel via the water-use assembly; and a gray water overflow container coupled in overflow receiving communication with the second variable volume fluid vessel.