For those who want to get out on the water on something other than a cruise boat, this sailing boat (advertised as a sailing ship, I think) has 2 sailings a day, though when we saw it out in the harbour, it looked like it was under engine power rather than sail....
I totally recommend this trip, which we took with Canterbury Cat (there are other operators doing them too but these guys came specially recommended to us, and we were very happy with the trip & service).
A part of the money raised from the cruises goes back into helping these Hector Dolphins, as they are on the endangered list.
Whether you're swimming with them or just watching from the boat, this trip is just great.
I spent about as much time in the boat as I did in the water, instead of just being in the water, as it was often hard to see them in the water, unless they surfaced a couple of metres from you (and after they'd gotten used to us, they were coming in even closer than this). From above, we were able to see them very clearly as they swam on our bow wave 3 at a time as the boat chugged along (they're not fast swimmers, evidently) and also better to see them just under the surface, which you can't see from in the water at all, as the water clarity isn't too good.
Equipment: Don't need to bring any gear - snorkel and face mask is supplied, as are the wetsuits.
You get kitted up at their large boat shed on the pier, and for the snorkellers there are hot showers when you get back.
Highly recommend the trip! One of the top 3 things we did on our whole New Zealand holiday.