The volume of water is as good as the shape of the reservoir (container) it’s in. Often, the containers have a circular, rectangular or a square cross-section.
Water In a Cylinder
If the water is in a cylindrical container, then the volume of that water is calculated using the formula to calculate the volume of a cylinder.
Assuming the water is full, then;

If the water is only halfway high in the container, then you’ll need to use half the height of the cylinder in the formula.
Water In a Box Shaped Container
For a rectangular based container, we use the formula to calculate the volume of a rectangular prism also known as cuboid.

For a cube shaped container, we use the formula to calculate the volume of a cube.

Therefore, the volume of water or any other liquid for that matter, is calculated by finding the volume of the container holding the liquid.