Notwithstanding the difficulties previously mentioned, there are some reports of success using ultrasonic distance sensors (intended for use in air) to measure distances under water:
"Seawolf" describes using an ultrasonic sensor for underwater depth sensing: "Preliminary results in the swimming pool show that it can measure from 4 to 12 feet pretty accurately."
"Admin" in this link and this one describes success with a waterproofed MaxBotix sensor: "The home-waterproofed MaxSonar that I did worked really really well. I tested it in a large cylindrical nuclear test plant vat of water that's 50 feet deep and 50 feet diameter (5 stories both directions). The MaxSonar was able to "see" the entire tank. "
This paper describes apparently successful use of a MaxBotix product as an underwater distance sensor.
MaxBotix has a page that talks about some of the challenges related to using their sensors underwater.
And this one seems to suggest success as well.