Hi, I was thinking about a similar project, using the sensor as it is won`t work for sure. (or 99%) my plan was to create a custom sonar sensor using the famous piezzo drum discs, but it require some additional circuits and complicated signal processing so i have dropped the idea for now. what is may be possible but haven`t got to it yet and probably not in the close future, is to disamble the hc-sr04 sensor and use its board and actuators separately. sinking the transmitter/receiver as they are underwater (will be damaged by corosion but as long as it is not sea water shouldn't short circuit).
another option is just to buy a waterproof sonar seen one on ebay for 15$
anyway take in count that underwater the speed of sound is different and has different decay, so results will differ (don`t remember numbers).
hope you will have some results to share