I have the 898 si and the si allows you to find offshore structure NOT on any map. Cruise a likely area with the si and drop waypoints on structure. You can then go over them with convetional sonar to see if it is worth fishing. I usually give it a shot just in case I missed marking them.
When fishing big water no amount of bouys, towers and landmarks are going to put you on structure that can be as small as the hood of a car, imho.