Hey Karan,
The SonarMote kit from LPL has this acrylic lasercut case, you can pretty easily "waterproof" it with various more or less intrusive methods, but I would still not consider it a long term proofing. I service my sump pump SonarMote about every 1-2 years, sometimes I find small amounts of corrosion that I clean up. Then I re-tape it all around in clear tape but not around the sensor openings.
The sonar sensor itself cannot really be waterproofed, and it has a limited range, works at 5V (requires a little booster, included on the SonarMote board).
I do not know of another reliable low power sensor to reach 20ft. I would try making some kind of resistive sensor for such an application.