It will not hurt the unit to leave it powered on out of the water ...
It may take the unit a few seconds to find bottom again after the TM is put back into the water ...
You might think about simply turning of the Sonar before raising the TM and then turn Sonar back on when deploying the TM ... It may find bottom quicker this way ...
To turn the Sonar off ... :
Press the power button one short press ...
Choose "Sonar Off" ...
Rickie