New Zealand isn't one of the world's great SCUBA diving locations, particularly in comparison to its South Pacific neighbours. It's too far South to allow the growth of coral reefs and the colourful inhabitants they attract. About the only diving area of any note is at the Poor Knights Islands, East of the Northern end of the country's North Island.
However, don't think that there's nothing worthwhile along New Zealand's enormous coastline. There are lots of interesting maritime bird species, like albatrosses, gannets and penguins, all of which have nesting colonies which can easily be visited by even casual tourists. There are also marine mammals like whales, dolphins and seals which can be viewed just as easily.
Plenty of places around the world allow you to swim with sharks, whales and dolphins, but where else in the world are you going to swim with the krill? Every year, vast numbers of these small crustaceans congregate along the coast of the South Island, particularly near Kaikoura on the North-East coast and the Marlborough Sounds on the Northern coast. Very few New Zealanders are even aware of this event unless, like me, they happen to stumble upon it. It was just a lucky break for me that I happened to go to the Marlborough Sounds town of Picton to catch the ferry from the South Island to the North Island, when I was moving house from Christchurch to Wellington. When I got to Picton, what did I see but large schools of bright red krill against a green background, looking all the world like grossly miscoloured oil slicks on the water's surface. It was also a fortunate thing that I had my underwater camera with me, and plenty of time to take photographs of the krill both from above and below the surface.