How Clear Is the Ocean During Obon (Aug 10–18) in Japan? Real Data Analysis

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

Is 'Obon Ocean Is Clear' Actually True?

Obon (August 10–18) is one of Japan's biggest summer holidays. Both beachgoers and divers flock to the ocean during this period, but is the image of 'crystal-clear Obon waters' actually supported by data? We compared real visibility observations from 11 dive sites between the Obon period (Aug 10–18) and the rest of August.

Bottom line: 6 of 11 sites show better visibility outside Obon than during it. There's no data suggesting Obon waters are unusually clear.

Obon vs Rest of August (11 Sites)

Site	Region	Obon (10–18)	Rest of Aug	Diff
Yonaguni	Okinawa	25.7m	26.6m	-0.9m
Kerama	Okinawa	21.3m	20.8m	+0.5m
Ishigaki	Okinawa	20.3m	21.2m	-0.9m
Mikomoto	Shizuoka	12.7m	12.8m	-0.1m
Kushimoto	Wakayama	11.6m	12.8m	-1.2m
IOP	Shizuoka	11.4m	12.5m	-1.1m
Kumomi	Shizuoka	10.4m	10.1m	+0.3m
Futo	Shizuoka	9.4m	10.2m	-0.8m
Echizen	Fukui	9.4m	10.2m	-0.8m
Hirasawa	Kanagawa	7.5m	7.2m	+0.3m
Aoumi	Yamaguchi	10.7m	10.6m	+0.1m

Red = Obon is lower; green = Obon is higher. Maximum difference within ±1.2m.

Breaking August Into Early, Obon, and Late Periods

Site	Early (1–9)	Obon (10–18)	Late (19–31)	Note
Yonaguni	26.3m	26.3m	26.7m	Stable throughout August; no difference
Kerama	21.4m	21.1m	20.4m	Early August slightly better
Mikomoto	13m	12.5m	12.6m	Early August is best
Kushimoto	13.6m	12.3m	12.1m	Early August peaks at 13.6m
IOP	12.5m	11.6m	12.6m	Obon is the lowest! 0.9m below early/late
Echizen	9.8m	10.4m	10.6m	Improves toward late August
Futo	10m	9.8m	10.4m	Late August slightly better

Obon period (10–18) highlighted with orange border

Why Do Some Sites Show Lower Visibility During Obon?

Notably at IOP (Izu Oceanic Park), the Obon period (11.4m) is about 1m lower than both early (12.5m) and late August (12.6m). Several factors may contribute:

Plankton growth near water temperature peak: Mid-August water temperatures are at their annual maximum, temporarily favoring phytoplankton growth
Rain and typhoon effects: The typhoon season intensifies around Obon, with rain and wave turbidity becoming more likely
Sediment disturbance from increased divers: The concentration of divers during Obon may stir up bottom sediment, reducing visibility (especially in shallow areas)

However, the differences are at most 1–1.2m — Obon conditions are far from 'undivable.' The tendency is simply for slightly lower clarity within August, not a dramatic deterioration.

Okinawa: Minimal Impact

Yonaguni maintains 25–26m stability throughout August, with Obon differences under 0.1m. Kerama even shows slightly higher Obon clarity (21.3m). Okinawa's strong oceanic water influence maintains high visibility regardless of time period — Obon doesn't change the quality of 'Okinawa summer' diving.

Practical Advice for Obon Divers

Go to Okinawa if visibility is your priority: Yonaguni/Kerama maintain 25–26m with minimal Obon impact
For Izu Peninsula: early August (1–9) is slightly better: IOP and Kushimoto both peak in early August
For Sea of Japan sites: late August (19+) is best: Echizen and Aoumi improve toward late August post-rainy season
Don't expect extra clarity just because it's Obon: Data doesn't support this; if anything, visibility is slightly lower

Summary

The image of 'clear Obon waters' isn't supported by data. At 8 of 11 sites, the rest of August shows higher visibility than the Obon period, with differences of about 1m at IOP, Kushimoto, and Yonaguni. However, the differences are small — even during Obon, Okinawa delivers 20m+ and Izu provides 10–13m. It's not the Obon period itself that determines visibility, but site selection and weather conditions.

Data source: scraped from public dive logs. Obon = Aug 10–18; early = Aug 1–9; late = Aug 19–31.