I think you got the model # wrong on the 898. That 898 ci model would indicate an internal GPS which is not availiable on the 898.
(In the Humminbird model #, the C indicates it is a combo sonar/GPS unit, the i indicates it would have an internal GPS, SI would indicate it has side imaging capability, in the 898 and 998 the HD designation would show it is the 2013 model with the new LED backlit display.)(HD for the 798 would indicate that it is the newer 798 model with ethernet. If no HD on the 798, it doesn't have ethernet.)
Assuming your 798 model # is correct, it has the internal GPS. It is also available without the internal GPS.
You want the GPS antenna as close to the transducer as feasible when marking waypoints. The marked waypoint accuracy gets poorer the further the GPS antenna is from the transducer.
I am getting very long winded here, but it is important to understand what happens when you mark a waypoint. Assuming you are marking an object when you are directly over it with the sonar (transducer), the waypoint records where the GPS antenna is at the instant you mark the waypoint.....not where the transducer is. Therefore, the closer the GPS is to the transducer, the more accurate your waypoint will be.
If the 798 is on the bow, and you want to use the 798 to return to a waypoint previously marked with the 898 which has it's transducer and GPS on the transom....using GPS only, the internal GPS in the 798 would be good......The best way to go would be to have a transducer for the 798 mounted on your trolling motor and use the GPS in the 798 at the bow for marking and returning to waypoints, and the 898 transducer and GPS mounted on the transom as close as feasible.
You could share the transducer and GPS antenna from the 898 and use it on the 798, but if the 798 is on the bow, you would be a boat length away from your waypoint.
If it still hasn't clicked for you, search for waypoint accuracy, and you should be able to find more info and some visual presentations that may help.