I have heard of the warning that the transducer can be damaged (by overheating) if operated out of water for a prolonged period. I have never heard it causing damage in practice however.
I think the risk is mainly to higher powered fish
You can test the depth sounder on the hard
by placing a large bowl of water on the ground underneath the transducer (note the transducer is still in air). The water provides a flat reflective surface, a lot of power is lost
in the air / transducer interface, but often enough remains to get a reading.
The speed in air and water is different so you need to multiply the "depth" by a correction factor.
I installed 2 new depth units on my yacht 18 months ago (one as backup) and gave the bowl of water a try, before launch. It only worked for one unit, but both functioned OK in the water.
Of course a very deep bowl in contact with the transducer would work better to test the unit, but as most instrument have a minimum reading of around 1m it is hard to find a suitable container to hold the water. The beam also spreads out and unless the container was large reflections from the side would cause problems.
I think damage is very unlikely, but try the bowl on the ground trick if it reads correctly you will at least be reassured.