You can connect it to the 998c SI via Ethernet and share GPS data and compatable accessories.
You would need the AS EC QDE Ethernet adaptor for the 859 and an Ehernet cable long enough to connect the units along the path you route the cable.
I would suggest that you exchange the unopened transducer package for the XTM 9 WIDE DI 20 T trolling motor puck. It will give you better 2D performance and has extra internal shielding for that application.
Contact Humminbird Customer Service for that exchange.