There are more details about the HC SR04 schematic and modification over here
https://uglyduck.vajn.icu/ep/archive/2014/01/Making_a_better_HC_SR04_Echo_Locator.html
Instead of going through the trouble to make waterproof transducers work with HC SR04.
I just went for this other waterproof sensor one instead which has 2 transducers therefore less minimum distance and is sealed making it waterproof from all sides.