gotta_fish,
If that is a 788ci Combo model unit than your only linking choice is the InterLink module.

If it is instead an “HD” version 788ci HD Combo model than you can Ethernet connect them via an Ethernet Connection Cable plus the 700 series Ethernet adapter cable.

Basic differences in the types of links are as has been explained: both Ethernet connected units have to be on to share the data, however waypoint data is not copied to the other unit so you can save more waypoints versus with the InterLink. With the InterLink when a waypoint is marked it is saved in the InterLink and then sent to the other unit when it is powered on (or if it is already on). This creates another copy of the same waypoint on the other unit which can use up available storage space on the unit. InterLink waypoint maximum storage is limited by either the InterLink or by the unit with the smallest waypoint storage capacity. With the Ethernet the other unit is treated as an external hard drive and the waypoint data is read from the other unit and those waypoint icons are displayed as a different color on the other unit. Waypoint storage is limited to the maximum storage capabilities of both units combined.

Only two units may be InterLinked at the same time.
Many units and accessories plus sonar data can be shared with the Ethernet connection.