Best Marine Garmin Gps Map For Boating On The Upper Mississippi River

I recently bought a boat with a Garmin GPS unit installed. I looked it up, and it is a $900 unit, but the tracking and detail are terrible.
I boat near LaCrosse WI, and according to the GPS unit, half the time I am on dry land. Garmin says they do not have any software that is any better. Has any one purchased a add on map that works on the upper Mississippi river ?

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