I took the boat out to try and answer my question , didn't get the result I expected. What I found is the NMEA output depth is the actual water depth and the 997 depth offset had no effect , but it does change the unit digital depth readout. Surprisingly the depth offset appears to effect the 200 khz beam recording , the depth is the offset depth. I find that hard to believe. Unfortunately , I needed to winterize and putup the boat for the season. Hopefully someone down south can take a test .