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Actually, most depth transducers are about 50k Hz frequency - some are about 200 kHz, and some are both ( ones with a shallow/deep setting) the transducers are tuned to be coupled to water. There are transducers that work in air, but they are of lower frequency, because it takes more energy to transmit through air. At 50 kHz the depth transducer COULD probably transmit and receive through air, but it would need a much higher energy signal, which the depth sounder power supply can't deliver.
It's best wait until the boat is the water to check the xducer. The "bowl of water" idea will not work because there is deadband or a minimum depth that the unit wants to see. Unless maybe a really deep bowl.
Leaving it on in air won't hurt it in the short run, but over time the transducer can mechanically wear out. the surface of the transducer is actually moving 50,000 times a second - it's not moving very far - maybe a couple of microns - but it still moves and can wear.