That's right thanks. I am reading the raw value of 2nd and 3rd byte and I think I'd have to read something different from zero but maybe I am wrong.
Do you have it in water with a target in front of the sensor?
If not then you will get zero.
I use this commercial pressure sensor to measure the level of water in storage tanks. They are fully calibrated, certified, and function reliably for decades.
what makes the IP68 sensor "realizes" that it is underwater?
It does not. The transducer is designed to efficiently transfer sound energy to water, and simply does not work in air. For the same reason, transducers designed to transfer sound energy to air do not work in water.
Air is compressible, so sound is transmitted by waves of compressed and rarefaction air. But liquids are NOT compressible, so the sound wave is produced by actually MOVING the water to propagate the energy. Think of water hammer in your house plumbing if you have steel pipes.
For air the ultrasonic device can receive as well as transmit. For underwater, the receive sensor is not the sender. At least on the bigger devices.
If you have access to the exterior bottom of the tank, you could also use a sensor such as the DS1603L mentioned in this post. That one has certain limitations on tank wall and fluid depth, but there are probably others of that type with different specs.
How to read serial data from Non-contact Ultrasonic liquid level sensor - #13 by wvmarle