[somesaba]

The robot arm is expensive because it needs precision hardware (backlash free bearings/gearing).

Your idea also requires precision hardware to move the sphere.

[ccozan]

Actually besides 3 step motors ( for rotating in three axis ), rubber wheels to hold the sphere and a sensor to measure the water level, I don't think you need more. What I don't know is how much water you need, probably this can also be computed.

It's a better idea than dipping with a robot arm. And also faster.