The best way to test the effect of mass on kinetic energy in this scenario is to:
drop two balls with different masses from the same height into water and observe the size of each splash.
This method allows the student to directly compare how the different masses affect the kinetic energy upon impact, as kinetic energy is directly related to mass and the square of velocity. By dropping them from the same height, the initial velocity will be the same, ensuring that any differences in outcome can be attributed to the differences in mass.