The downside to this method is if you have the same points on the HB 587 and the Garmin 76 it will duplicate those so you will have 2 points for that spot. Simple fix is delete one on the water when you are at the spot and see 2 points.
BTW.
You can enter the GPS points manually in the 587.
Go to waypoints submenu and select create press the right cursor key then simply enter the lat and long and name the point.