Okinoshima (Kushimoto) Visibility — 1.2m Clearer Than Mainland Kushimoto

2026-03-15

Okinoshima sits about 10 minutes by boat from Kushimoto, at the southern tip of the Kii Peninsula. Known as the area's premier open-ocean point, our analysis of 218 real observations reveals an annual average of 13.1m — 1.2m above Kushimoto mainland (11.9m). In winter (January), it reaches 18.2m vs mainland's 13.8m.

13.1m

Okinoshima avg

11.9m

Kushimoto avg

18.2m

Jan avg (peak)

218 obs

Observations

Monthly Comparison: Okinoshima vs Kushimoto Mainland

Month	Okinoshima	Kushimoto	Diff
Jan	18.2m	13.8m	+4.4m
Feb	15.6m	12.5m	+3.1m
Mar	12.2m	10m	+2.2m
Apr	12.3m	10m	+2.3m
May	13.2m	10.8m	+2.4m
Jun	No data	12.2m	-
Jul	No data	11.5m	-
Aug	12.2m	12.7m	-0.5m
Sep	12.6m	11.5m	+1.1m
Oct	13.1m	12.9m	+0.2m
Nov	13.1m	11.6m	+1.5m
Dec	13.5m	13.1m	+0.4m

Note: Okinoshima has no June–July data, and winter months (Jan, Mar–Apr) have only 4–6 observations. Summer–autumn (Aug–Nov) data with 42–54 observations is more reliable.

Why Is Okinoshima Clearer Than Mainland Kushimoto?

Open Ocean Exposure

Okinoshima's offshore position receives the Kuroshio Current more directly. The Kuroshio carries nutrient-poor, clear oceanic water — offshore points tend to be clearer.

Less Coastal Influence

Mainland Kushimoto is more affected by river inflow and coastal sediment, while Okinoshima is surrounded by deep water, reducing these effects. The difference is especially noticeable after heavy rain.

Diving Okinoshima: What to Know

Okinoshima is boat diving only with sometimes strong currents — intermediate+ recommended. Boats may not run in rough conditions. Best seasons: winter (Dec–Feb, 13.5–18.2m) and autumn (Sep–Nov, 12.6–13.1m). Known for coral colonies and pelagic fish encounters.

About the Data

From Kushimoto area dive shop daily logs. Okinoshima: 218 obs (Aug 2019 – Nov 2025). Kushimoto: 3,172 obs (Jan 2016 – Mar 2026). Visibility is mean of daily min/max. Reference: Kushimoto Marine Park Center long-term monitoring data.

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