I have 2 Garmin UHD 93sv units which came preloaded with US inland lakes.  Last week I fished the Susquehanna Flats for the first time which is 20 miles from my house.  That portion of the river is tidal and is at the top of the Chesapeake Bay.  I was surprised when depths were not shown on any of the maps as they are with the lakes I've fished.  It appears I need to get coastal maps for the added detail (US East, Lakes, Rivers and Coastal Marine Charts).

 

Has anyone purchased new maps from Garmin?  I there anything I need to watch out for?  Would this purchase overwrite the maps currently on my units?  Have you downloaded maps using the Active Captain App?

I've purchased a lot of Garmin maps, always on line through Garmin to insure they're legit.  The new chart does not overwrite the chart in the unit.  If you remove the micro sd card, your unit reverts to the built-in chart.  

 

I found using Active Captain was a pain but I guess it's the future.  You can purchase a sd card and avoid the download.  One thing I found for NC coastal rivers is that there is not a lot of depth information available.  The major channels are marked but other than that, not that much.  I used the Quick Draw contours to help out, keeping in mind the tidal changes in depth.