Refraction vs Reflection in Ultrasonic sensor for water level detection
You can neglect the part of sound intensity entering the water.

What happens at the surface is illustrated here:

(Source: https://de.wikipedia.org/wiki/Datei:Partial_transmittance.gif)

The more different, however, wave impedances of both media (air and water) are the more energy is reflected and fewer energy enters the second medium.

At a perpendicular angle of incidence the ratio of reflected and incident sound intensity is

\$\frac{I_r}{I_i} = (\frac{Z_{air} - Z_{water}}{ Z_{air} + Z_{water}})^2\$

where
\$Z_{air} \approx 410 kg/m^2s\$ and
\$Z_{water}\approx 1480000 kg/m^2s\$  

(waveimpedance values are taken from German Wikipedia article about waveimpedance)

As you can see, the values for air and water are very different and therefore the ratio is very very close to 1 (ca. 0.999), i.e. almost all sound intensity is refelected and almost none is entering the water.

BTW: this is why a gel is applied between source/sensor and skin when doing ultrasound scans. Otherwise, if there was even a thin layer of air between sound source/sensor and body most of the ultrasound energy couldnd't enter and leave the body (\$\approx\$ water).