Here's a back-alley way to do it:
You could select different waypoint symbols of the different depths you're interested in
(5, 10, 15, 20, 25, etc). Each depth will have a different waypoint symbol.
Make your sonar recording(s).
Then, as you're playing back the recordings, freeze the frame and press the mark button
for each depth you want to save. I guess it would be best to place the cursor at the
lake/river bottom before pressing mark. The initial waypoint symbol is a blue circle.
Make sure you write down each waypoint number and the depth every time you press
the mark button. It will display the number of the new waypoint on the screen for a second.
Then, after reviewing and closing out the recording, go back to each waypoint and edit it and change the symbol.
Those waypoints will show up on your chart.
I know it's raw, but it's better than nothing.
Good Luck.