The 785c2 is a GPS/sonar combo The way I understand it, the system is ready to use a transducer. As long as you purchase a unit with a tranducer, you have complete sonar (fishfinding capabilities). The unit is preloaded with the software necessary to use the data from the transducer to show fishing icons and alarms. If you purchase without the transducer you just have chatplotting capability.I'm not sure if the 785 is a fishfinder or sonar or what. Were you able to confirm what each model does by the manual?