As well as a two-hour nature cruise, Black Cat offers a three-hour 'swimming with dolphins' experience. Wetsuits and snorkelling gear are provided, plus hot showers back on dry land. Observers can tag along (adult/child $95/55) but only 12 people can swim per trip, so book ahead.
Cruises have a 98% success rate in seeing Akaroa's compact Hector's dolphins, and an 83% success rate in actually swimming with them (there's a 25% refund if there's no swim).
Note that while concerns over the impact of swimming tours on the bottlenose dolphins in the Bay of Islands have caused such tours to be banned there, they are still allowed here in Akaroa.