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.
A common metric unit to measure a bird bath would be centimeters (cm) or meters (m). These units can help determine the dimensions and size of the bird bath accurately.
Which unit would you use to measure the water in a full tub?
Liter
Gallons
You would use the metric unit "liters" to measure the capacity of a hot water tank.
A common unit to measure water in a bath tub is liters. This unit provides a convenient way to measure and control the amount of water being used for bathing. Another commonly used unit is gallons, especially in countries like the United States.
Litres
The unit typically used to measure the amount of water in a fish tank is liters (L) or gallons (gal).
The metric unit commonly used to measure the depth of water in a bath is centimeters (cm) or meters (m).
gallons
Litre