I understand your frustration ...as there is just not much information in the advertising specs of what the Unimap and ContourXD base map in the HB units actually consist of...(especially with the vast amounts of researching one must do when trying to decide on such an important and expensive undertaking)...
The 598ci HD SI Combo comes standard with supplied Unimap...which is basically just lake outlines that saved waypoints will be transparently imposed over...
The 700 series and above chartplotting units come standard with supplied ContourXD...which is a mere 3000 covered lakes with some contour detail ...and all other lakes in the US of just lake outlines as explained above like Unimap...(the US alone has tens of thousands of lakes - it's unlikely anyone's home lake will be 1 of the 3000 included in the ContourXD map data)..
HB stopped offering built-in cartography (other than the ContourXD mentioned above)...with the cessation of the NVB models somewhere around 2011...(I think)...
All HB chartplotting units since that time can be coordinated with the aftermarket charting map cards by Navionics and Lakemaster....
Each have their advantages depending on your home lake mapping data needed...
Navionics covered lakes...
http://www.navionics.com/en/hotmaps-premium
Lakemaster covered lakes...
http://www.lakemap.com/Find-a-Lake/#.U1q4De69LYU
I realize that an additional "Ben Franklin + some" for a mapping card is difficult some times....but it will be worth it...
There are occasions that used map cards pop up in the forum's classified sections if you want to wait and try to save a few bucks buying a used map card...
Rickie