Liters would be the metric measurement for volume of liquid.
The common metric unit of volume is the liter, which would be appropriate to measure this amount of water. The SI unit of volume is the cubic meter, though.
Millilitres (mL) would be the best metric measure for a cup of water.
What is the metric unit of a orange? What is the metric unit of a pencil? What iw the customary unit amount of water a bathtub will hold?
Which unit would you use to measure the water in a full tub?