Turns out, the depthfinder has to be set to "advanced user mode" in the Settings menu before you have an option to delete nav data in the Navigation menu. I saw the suggestion to "delete navigation data" in a thread for an 8XX series depthfinder, but didn't think that was an option on my 788 series since I didn't see it in the Navigation menu.
When I got my 788ci, the first thing I did was set the user mode to advanced. But, I had restored the factory defaults in anticipation of a firmware update, which returned the user mode to "normal" and it never occurred to me that my fishfinder might not be in "advanced." You know what they about assumptions...
So, I guess the big tip is check your user mode, then you can delete navigation data.