As well as the Christchurch estuary (which hosts* a few
dinghy clubs) Christchurch has a harbor, Lyttelton, the keelboats
race out of Naval Point Club where there is(was) also a marina*, no idea if liveaboards are still allowed. Otherwise quite a few boats are moored in Diamond harbor or Purau bay, on the far side of Lyttelton. Akaroa harbour is also very nice, but a bit far to commute to Christchurch every day.
*I've only been back briefly since the 2011 earthquake, so I don't know which have closed and which were rebuilt, but the marina was badly damaged.