Stickman,
The 788ci HD Di model cannot share sonar data with the 998c Si unit. They use two different forms of Down Imaging sonar. So the sonar data for the 788ci HD DI will come from its own transducer. If you want to mount the DI transducer on the trolling motor you need to exchange the one in that comes with the unit for an XTM-9-DI-25-T model transducer. XTM 9 DI 25 T
The folks at Humminbird can help you with that (1-800-633-1468). The 788ci HD Di unit can still share waypoint data with the 998c Si unit.

If you went with the 788ci HD unit it can share the 2D sonar (200kHz/83kHz) with the 998c Si unit along with the waypoint data.