Solution

The sound gets stronger when the air column in the vessel above the water has length $L$ that permits a standing wave, i.e. $L=(2n-1)\lambda/4=(2n-1)c/(4f)$, where $c$ is the speed of sound and $f$ is the frequency of the tuning fork. In between, the sound gets weaker. The figure shows standing waves of displacement waves, which must have antinodes at the opening of the vessel and nodes at water surface. (The fact that the standing waves fit the width of the vessel is for an esthetic not physical reason.)