Sounds like you are considering a gps/sounder combo unit. I have been using the 917c by Humminbird. They usually come with a remote mounted gps antenna and a transom mount transducer. The type of transducer that works best depends on your boats setup. I have a full inboard engine so a transom mount transducer would not work well for me because of cavitation noise and turbulent water in that area. I upgraded to a thru-hull transducer which requires drilling a hole through the hull and mounting it in the mid section of the boat, This will give you the best readings possible. A third method is called inside the hull mount. This is okay for single layer fiberglass boats and requires you to epoxy the transducer in the bilge to the hull of the boat. The amount of transmitting loss depends on hull construction and thickness, as well as installation position and process. Transom mount transducers work well with outboard and outdrive engines.
Many transducers have dual beams, a 200khz and a 83khz beam this will give you a narrow and wide view. The lower the frequency beams are capable of penetrating greater depths, but with less resolution. I chose a quadrabeam transducer that uses a 200khz,83khz and two 455khz beams. The 455khz beams allow me to read the port and starboard sides of the boat so I get a full 90 degrees of coverage. This is usefull when jigging and determining the direction a school of fish is moving.
As far as maps go, the more you spend the more you get. I have been using Navionics gold which covers everything. Navionics Platinum is better with 3D graphics. And Hotmaps works best for lakes and rivers. All gps manufacturers usually recommend a map package that will suite your needs. You can research the websights and see the quality that would like for your boat.
