Well, here's yet another example where an ultrasonic distance sensor intended for air was successfully used to measure water depth: https://coconutpi.wordpress.com/ The section "Depth and Obstacle Detection" contains this image:
Image title "Our $US99 MaxBotix MB7078 in a waterproof housing interfaced with a TTL-RS232 Convertor and connected to an Arduino Uno"
And their test results:
The slope of the fitted line is "...4.17 which was close to the theoretical gradient of 4.31 (by dividing speed of sound in water by speed of sound in air both at 30 degrees celsius)"
Still not convinced?