For spearfishing, head for french pass (Stay away from the pass itself, it is safe to swim around the small beach to the west but not any closer) or port underwood (Visbility likely to be crap).
I have dived several times heading west of the pass and never seen anything worth shoting. There was a resident red moki for a while. Might be best to swim around to the cliffs on the eastern side of the township.
If you own a boat Okiwi bay is also worth while, but there is little shorebased around there.