Nestled on the east coast of New Zealand's South Island, Christchurch serves as an urban gateway to an array of marine experiences. The waters around Christchurch, particularly those near Banks Peninsula, offer a unique mix of temperate reefs, intriguing rock formations, and a diverse range of marine life.

Cathedral Rocks: Located near Akaroa on the Banks Peninsula, Cathedral Rocks offers intricate cave systems and swim-throughs that will enthral experienced divers. As the name suggests, diving here feels like exploring the naves of an underwater cathedral.

Corsair Bay: A short drive from Christchurch city center, Corsair Bay is a sheltered spot that's ideal for beginner divers and training dives. Its calm conditions and shallow depths make it a safe and enjoyable dive site for those new to the sport.

The waters around Christchurch teem with diverse marine life. Divers can expect encounters with blue cod, wrasse, butterfish, and the playful New Zealand fur seal. More elusive creatures like seahorses, octopuses, and nudibranchs can also be spotted on careful observation.

Dive conditions around Christchurch can be variable. The Banks Peninsula generally offers clearer waters than the open coastline. Water temperatures hover around 10°C (50°F) in winter and can reach up to 16°C (60°F) in summer. Visibility varies but is typically between 5 to 15 meters, with some spots on the peninsula offering up to 20 meters on good days.

The cost of diving in Christchurch will depend on the type of dive and whether equipment rental is needed. A two-tank dive generally costs between NZD 150-250. Equipment rental might add an additional NZD 50-100. Diving courses for beginners start at around NZD 500-650.

In conclusion, while Christchurch might be better known for its English-inspired architecture and serene Avon River, the underwater world nearby provides a contrasting adventure, replete with natural wonders and marine delights.