Golden Week vs Summer Vacation: Which Has Clearer Diving in Japan?

March 11, 2026 · 14 dive sites analyzed from real observation data

'Which Is Clearer — Even If Both Are Crowded?'

Golden Week and summer vacation (Obon/August) are Japan's two busiest dive periods. Prices are high and bookings are competitive. But if you're diving during these times anyway, at least let the data guide your choice. We honestly compare Golden Week (April–May) vs summer (August) visibility using real data from 14 dive sites.

Bottom line: Summer (Aug) wins at 11 of 14 sites. Golden Week is better at only Ishigaki, Ito, and Hirasawa.

GW vs Summer Visibility Comparison (14 Sites)

Site	Region	GW Early(Apr)	GW Late(May)	Summer(Aug)	Diff(Aug−May)
Yonaguni	Okinawa	23.9m	24.2m	26.5m	+2.4m
Ishigaki	Okinawa	21.2m	20.1m	21m	−0.2m
Kerama	Okinawa	18.6m	19.4m	20.9m	+1.5m
Ito	Chiba	14.8m	15.9m	15m	−0.9m
IOP	Shizuoka	10.1m	10.6m	12.3m	+1.7m
Futo	Shizuoka	9.5m	9.2m	10.1m	+0.9m
Mikomoto	Shizuoka	9.9m	9.7m	12.7m	+3m
Kashiwajima	Kochi	10.4m	11.5m	13m	+1.5m
Kushimoto	Wakayama	10m	10.8m	12.7m	+1.9m
Miyakejima	Tokyo	8.8m	9.2m	11.7m	+2.5m
Echizen	Fukui	7.6m	7.7m	10.3m	+2.6m
Aoumi	Yamaguchi	8.5m	8.6m	10.6m	+2m
Tajiri	Tottori	8.2m	8.7m	12.4m	+3.7m
Hirasawa	Kanagawa	8.4m	8.2m	7.1m	−1.1m

Positive diff = summer wins (blue); negative diff = GW wins (green)

Regional Summary

Region	GW avg	Aug avg	Verdict
Okinawa	21.2m	22.8m	Summer leads (+1.6m)
Izu Peninsula	9.8m	11.8m	Summer leads (+2.0m)
Sea of Japan	8.2m	11.1m	Summer strongly leads (+2.9m)
Chiba (Ito)	15.4m	15m	GW slightly better (+0.4m)

Why Is Summer Generally Clearer Than Golden Week?

April and May are Japan's 'spring bloom' season. As water temperatures begin rising, phytoplankton proliferates explosively, significantly reducing ocean transparency. This is a universal phenomenon along Japan's coastal waters, particularly visible at Izu, Sea of Japan, and Kii Peninsula sites. By August, stable high temperatures suppress nutrient cycling, reducing plankton and allowing clarity to recover.

Exception 1: Ishigaki (April is the Annual Best)

Ishigaki's April (21.2m) virtually ties August (21.0m), with April edging ahead. The Ishigaki area is strongly influenced by southern oceanic water, limiting spring bloom effects. Visiting Ishigaki during Golden Week is excellent from a visibility standpoint.

Exception 2: Ito, Chiba (May Is the Annual Best)

Ito in Chiba shows 15.9m in May vs 15.0m in August. Located at the entrance to Tokyo Bay, stable oceanic water inflow slows spring warming and suppresses plankton blooms. For Golden Week diving near the Tokyo area, Ito is one of the best choices.

Warning: Hirasawa (Kanagawa) Is Worse in Summer

Hirasawa in Kanagawa shows April (8.4m) and May (8.2m) vs August (7.1m) — summer is actually worse. Sagami Bay's geography may promote phytoplankton growth in summer. The data shows winter-to-spring as Hirasawa's best season.

Sites to Visit vs Avoid During Golden Week

✓ Good for Golden Week

Ishigaki: April is annual peak (GW May 20.1m)
Ito: May is annual best! (GW May 15.9m)
Hirasawa: Summer actually worse! (GW May 8.2m)
※Okinawa: summer wins, but GW still delivers 18–24m quality

✗ Better in Summer Than GW

Tajiri (summer +3.7m)
Mikomoto (summer +3m)
Echizen (summer +2.6m)
Miyakejima (summer +2.5m)
Yonaguni (summer +2.4m)
Aoumi (summer +2m)

Summary

The data verdict is clear: summer (August) beats Golden Week at most sites, especially Sea of Japan and Izu Peninsula sites where August delivers 2–4m more clarity. However, Ishigaki peaks in April and matches summer levels, while Ito (Chiba) actually has its annual best in May. If planning domestic diving as an alternative to overseas trips: choose Ishigaki or Ito for Golden Week; virtually anywhere for summer vacation.

Data source: scraped from public dive logs. Golden Week approximated using April–May monthly averages.