The answer depends on what aspect you wish to measure: its mass, its area, its depth, the volume of water that it can hold, etc.
Centilitres.
Which unit would you use to measure the water in a full tub?
You would use litres
Liter
Gallons
Litres
The unit typically used to measure the amount of water in a fish tank is liters (L) or gallons (gal).
Litre
Litre
gallons
metric unit
The best unit of measure to use when measuring the amount of water in a drinking glass is ounces. A liquid measuring cup can be used to measure the ounces of water.