Izu Peninsula — Osezaki & IOP

A day here looks nothing like a boat trip. Divers park behind the beach, pay the facility fee at the local diving association hut and trolley their gear down to the water, then repeat that two or three times with surface intervals spent in the shop. Guides brief off laminated sheets of what is currently on the slope, because the cast turns over week to week and the finds are marked rather than stumbled upon. Weekends draw a crowd out of Tokyo, so weekday diving is noticeably calmer.

Osezaki's sheltered side faces the inner bay: dark sand and silt, gently graded, with sunken ropes, markers and debris that critters colonise and guides revisit dive after dive. Round the tip of the spit the ground turns steeper and colder as it falls away toward Suruga Bay proper, the deepest bay in Japan, and clarity improves with it. Izu Ocean Park sits on the peninsula's other coast, facing Sagami Bay: volcanic rock, seasonal kelp, arches and pinnacles off a stepped shore entry.

What people come for is turnover. The Kuroshio pushes tropical larvae up the coast through late summer, and the juveniles that settle on the slope in autumn - Japanese divers call them shimetsu-kaiyugyo, roughly 'doomed migrants' - mostly will not last the winter, which is exactly why photographers time trips for them. The cold months swing the other way: the water clears, the shallow rock fills with nudibranchs, and dives get slower and more deliberate. Plankton can turn the water milky from spring into early summer and cut visibility to a few metres.

It rewards patience above all else. Divers happy to spend fifty minutes covering a few metres of sand with a guide who knows where a subject sat yesterday get more from this slope than most coastlines offer, and Tokyo shops run certification and refresher weekends on the same sheltered ground. Anyone hoping for coral gardens or big animals should look south instead; this coast trades them for detail.