You can buy upgraded maps that include "Google Earth" images - slick if you fish the marsh a lot. Navionics Platinum+ is excellent.
Anyhow, call Derek at Breakwater Marine in Houston and he can fix you up.
http://www.breakwatermarine.com/Default.aspx
I used Breakwater for my last 2 boats - solid, honest guys who know their stuff & no BS.
Good luck