I hooked up with an american guy who does a lot of spearfishing around the main Island (he lives in Tonga Tapu) and from there we went fishing from his boat and from the shore aswell. We'd pretty much shoot anything but sharks, rays and other endangered species, because the fish we didn't keep for ourselves he would sell to the locals. I was not experienced back then, but I did play a lot with the aspetto dives, where you go to the bottom and wait for the fish to approach you and then shoot them. It seems the most easy and effective way is to sneak up on them from above and go for a fast kill.
It is a couple of years past now, and back then it was small game, mainly parrot fish which they have in heaps, but also sea bass, trigger fish and other fish aswell. I've kept in touch with the guy on facebook, and it seems that now he has really stepped up his game, shooting fish ranging from 10-40 kg. He's sent me many impressive, jealousy-inducing pictures
I think theres definetely still a lot of fish in the main Island area. I went to the other Islands but that was without speargun, so I didn't hunt there. It looked good, though.
His name is Ben Carlson, and I believe he has a profile in here called coral_sniper. Back then I came with him for free, but he's perhaps made himself a business taking guys on tours on bigger boats, so he might charge you for tours, but I'm sure it will be worth every penny. He'll definetly be your go to guy, at least for recommending places to go.
I've written to him asking if its okay to pass on his contact info, just in case