As mentioned, koheru are usually encountered in areas where divers would be targetting kingfish, so pressure points where the current pushes on to large or rocky structures, are a great place to start hunting.
Koheru are usually found in very large schools and will often follow a diver thinking that he provides protection from predators. As long as your movements are slow and non-aggressive, they should be easily shot.
When shooting schooling fish, make sure you aim at a specific fish and don’t be tempted to just shoot randomly into the school and hope for the best as you’ll end up spooking the lot. With any luck, these schools will have attracted larger pelagic fish so it is in your best interests to keep them with you and not scare them off completely. Using burley and flashers are also good ways to attract koheru.