Hm, upon more research it seems that
Garmin uses an undocumented binary format in a file called UserData.ADM. Very nice of them. There is a shareware utility called, generically enough, "GPS Utility" that allows you to enter waypoints and create routes and export them to the SD card.
GPS Utility - Home
I am still playing with it but it seems that it will do the job. Unfortunately for me, I need a SD card formatted by the
Garmin unit to put the test data on, and my laptop can not write an SD card. So I either need to get a portable USB card reader or go to the boat, fetch the card, bring it home, write the date, then go try it out. I think I'll get a portable card writer.