The previous guide is incorrect.
For the achievement you must visit each of the "hoods" which are the purple outlined areas throughout the map. Flying over the areas counts as visiting them, so I would recommend a plane or helicopter if you are trying to get this sooner rather than later. The name of the district will be on the left side of the screen above the mini map so you can tell when you are over an area.
For me the last one I needed was Port Pryor at the south of the luchadores area. I would recommend checking for hoods you don't own any properties in as that's a good indicator you may have not visited that one yet.
I hope this helps save time for anyone like me who spent way too much time flying to each island multiple times trying to figure out why it wasn't popping.