You can make really good FAD's using bamboo and other types of wood and branches, you can get them to cover a really large area. Using chains set in cement and multiple ropes and buoys you can create a huge eco system. With the aid of divers you could maintain it and even add to the area.
You could set the Buoys so they are a couple of meters below the water at high tide. That will take care of coastguard worries.
If you set these at three or four locations several miles apart but close to Havelock on the 100 fathom line you could create world class Marlin fishing on your doorstep.
Presently you need to travel very long distances to the seamounts, too far for small boats to reach the Marlin fishing grounds.
By doing this you guys could realistically offer daytrips for Marlin.