The explanation is as follows.
When you add water to a bowl, the pitch or frequency gets lower. This happens because as you add water, you also add more mass to the bowl — the combined mass of the bowl plus the water. More mass results in a slower/lower vibrating frequency, and less mass produces a faster/higher vibrating frequency.
Also, the sound is lower because sound waves travel about three times faster in glass than they do in water. When water is added to a glass, part of the vibrating wave travels though the water instead of through the glass.
Hope it answers your question.
All the best!