can the JSN-SR04T waterproof ultrasonic sensor be used in underwater ?

My task is not to measure distance, my task is to have data communication underwater using Ultrasonic sensors, use one of these JST-SR04Ts as a transmitter on an arduino, and connect more sensors of the same kind to another Arduino and set them as receivers for the ultrasonic pulse that is coming from the first Ultrasonic transmitter, and since performance of ultrasonic waves underwater is better than air due to density increase in water thus it could have a theoretical increase in the range of the ultrasonic wave, what are your ideas on my project?, do you think it could work?, what problems could I face regarding this project?, thank you.

They're designed to work in air, and do not work in water.

Look at fish finders, boat sonar devices, or the DS1603L ultrasonic water level sensor. Those are designed to work in water (and consequently do not work in air).

what frequency does the DS1603L and what is the range that it can detect ultrasound from?

does it transmit and receive or is it only a transducer?, since it doesn't have echo and trigger pins

No idea about frequency; specs say 5m iirc. Do look it up if you’re interested in the sensor.

I saw a video about the DS1603L on YouTube but the guy used it outside the water not inside the water link: DS1603 ultrasonic level sender for electronic dipstick - YouTube
I want a sensor to transmit ultrasonic waves underwater and that functions underwater for both transmitting and receiving also if possible measure distance.
please help me anyone.

As already stated, the JST-SR04T won't work underwater. You will need a transducer specifically designed to be submerged in water, and the associated electronics.

For distance sensing to 100 m, cheap fish finders like this one work fine (I have one), but they are not Arduino compatible.

For data transmission, you will need two underwater acoustic modems, and they are quite expensive.

The DS1603L is designed to work through the wall of a container, but woks just the same when you hold it in the water. It will measure the distance between the sensor's front and the surface that's in front of it (be it the wall of the container or the water/air surface).