There was a previous post on this subject but it was several months ago with zero replies. He was trying to bring the data into DrDepth. I ran into the same issue when moving my data from Garmin to HB. Perhaps the below will help.
First I opened the Garmin MapSource file and save-as a .gpx file. When bringing the .gpx into HB PC all the depths displayed as 0.0 even though I knew there was valid depth data in the .gdb file. After searching the net for several programs to read .gpx files I determined the depth in the .gpx file actually was 0.0. Bad data exported from Garmin.
Next I saved the MapSource file as a .txt delimited and opened with MS-Excel. The depth was there but the file was a bit cluttered and besides there was no apparent way to get a .xls (Excel) or .txt file into HB PC.
Finally, I opened HB PC and "Close All" to clear current data. Then File/Open but this time, choose file type .gdb from the drop down File Type box. There are File types for Garmin and Lowrance. HB PC will display a screen about automatically creating a .gpx file, click OK to continue. The file will open and this time the depth will be displayed.
Good Luck and Tight Lines.