“How do I get my waypoints on my 587ci to my 898c SI?”
By saving the waypoints to an SD memory card (Export All Nav Data function) and installing the same SD memory card into your 898c Si unit.
“Will transferring waypoints delete the ones I currently have on my 898c SI?”
No, it will add all of the non-duplicate waypoints to the ones your 898c Si unit already has. A duplicate waypoint is one that has the same coordinates and name.
“One more question..... Do any of you all use the Navionics software to manage waypoints on your units?”
Nope, not me.