Garmin protects their software so it only works with their maps. Any discussion on how to get around that is not allowed on this site.
You CAN download the latest WM Garmin software free from their support site - just no maps.
You can use the CF-QUE product (a few years old now) with free maps from the internet:
Garmin: cf Que 1620 Updates & Downloads
Some free maps are here:
GPS Maps I used this on my last cruise.
Read the GPS forum at this site to find out how to use the GPSproxy software you need with CF-QUE.
Good luck.