I have often wondered about this myself. ???
I leave my depthfinder on while drop-shotting however, and catch fish while watching my lure on the screen. Have even seen them come and take the lure (on the screen). So it doesn't seem to bother them all that much - at least not all the time.
I wouldn't be running it in three feet of water, however, so don't have any idea of the effect on fish in shallow water like that.
TRK