Many echo sounders can be set up to read either ‘depth below transducer’ (generally the default reading), ‘depth of water (from the surface)’ or ‘depth below the keel’. Some people prefer to know that when the sounder reads zero they are aground. When reducing to soundings in fog, however, it is preferable for the instrument to read out the actual depth of water, since this the ‘number’ you will be calculating. When anchoring at mid-tide, you will also be operating from the actual depth of the water when working out low water clearance and scope.
To add or subtract a value for transducer depth every time supplies an extra potential for a mistake, so why not calibrate the beast for total depth of water and be done with it? After all, you will still know when you’ll run aground. It’s the same figure as your draught!