Idea 1.1:

Regarding the previous idea, we initially thought of using a colorimetric sensor and have the liquid be some shade of grey so if more liquid is added, the blacker the solution.

A humidity sensor was also an option, however since this game should be designed to be played several times, the container would not have time to dry off completely between plays. So this is not the best option.

We also had the idea of using a movement sensor which would detect if the water reached a certain level. However, as we are adding the water, the sensor would sense the falling water as movement. 

So the pressure sensor seems the best solution.