Why is it not possible to use underwater waterproof product ....?
Waterproof doesn't mean you can use it under water. It just means that it can be put under water without destroying it. Sonic distance sensors that are made for under-water use are usually called sonar but you'll find more on the internet if you use the search term "boat depth sensor".
Is it because the ultrasonic voltage is only 5V and the ultrasonic intensity is weak?
No, it's because these sensors are made to sense the first non-gaseous material in the beam direction. In your test it's the water. These sensors are often used to measure the liquid level in a tank, so they must handle liquid as an object.