Harry,
There would need to be some way to mark waypoints with the SI. Even if you have another GPS, you would still need to patch the GPS data in to the SI unit. And in order for it to work, the SI unit would still need some GPS capabilities. You could eliminate the Chartplotting, but GPS is a must. So, if GPS is already there, no sense in not having the chartplotting. They could still use their other unit to navigate like I do. I never change the 997c from the SI screen when searching and use the 785c2 to navigate.
The Side Imaging is really useless if you cannot get back to points of interest!
Robert