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It's like Boyd described, but the link to the preloaded Marine file for the original 8MB Legend is here:
http://www8.garmin.com/software/PreloadedMarinePOIDatabaseforeTrexLegend.exe

The description is as follows:
Download Preloaded Marine POI Database for eTrex Legend
If MapSource data is loaded to this unit, this unit's preloaded Marine POI database will be erased. Please download the file below to recover this Marine POI database. If you currently have MapSource data loaded into your GPS unit, it will be erased and replaced with this POI database.
http://www8.garmin.com/software/PreloadedMarinePOIDatabaseforeTrexLegend.exe (2.32 MB)

Looking at that same spot using the Garmin US Recreational Lakes with Fishing Hotspots V5, the area that you indcate is labeled as "submergent vegetation".  It is almost a perfect match with the green and black area in your screenshot.