Izu Islands 3-Way Comparison: Visibility Data for Oshima, Miyakejima & Hachijojima

Three Completely Different Worlds

The Izu Islands stretch south from Tokyo. While Oshima, Miyakejima, and Hachijojima are all accessible dive destinations, their visibility, water temperature, and seasonal patterns are completely different. Oshima (Akinohama) offers stable year-round visibility, Miyakejima boasts Japan's warmest waters packed with tropical fish, and Hachijojima records a remarkable 21.9m visibility in September. We analyze 2,571 real observations to reveal the differences.

3-Island Summary Comparison

※Best values in bold blue. Hachijojima data is mainly from 2025 onward — treat as indicative.

Monthly Visibility (All 12 Months)

Darker = clearer water (≥20m dark blue, ≥15m light blue, ≥12m pale cyan, ≥8m yellow)

Oshima (Akinohama): The Stability Champion

Annual average 14.3m. The standout feature is an extremely stable seasonal range of just 1.8m. January: 15.1m; April: 13.6m — the best and worst months differ by less than 2m. Even in midsummer, visibility stays at 13.8m, making it a year-round destination. Water temperature peaks at 23.8°C in summer and drops to 16.5°C in winter — similar to the Izu Peninsula. Its reputation as 'the stable island' makes it popular with divers of all levels throughout the year.

Notably, winter (December–February) delivers the year's best visibility. As Kuroshio influence weakens and plankton density drops, clarity peaks at 15.1m in both December and January — the annual maximum.

Miyakejima: Tropical Warmth, Temperate Clarity

Annual average visibility of 10.3m is the lowest of the three islands, but annual average water temperature of 24.9°C is by far the highest. September water temperature reaches 27.2°C — on par with Okinawa and Ishigaki. This warm water pushes the northern limit of tropical species like cleaner wrasses, regal angelfish, and fairy basslets, making Miyakejima Japan's closest tropical fish destination to Tokyo. Visibility in August–September is only 10–11m, but the experience of being surrounded by tropical fish is unavailable elsewhere in the Izu Islands.

The best visibility is surprisingly in December (12.8m), when water cools to around 21°C and plankton decreases. However, December has only 4 observations, so treat it as indicative rather than definitive.

Hachijojima: The September Miracle — 21.9m

The highest annual average of the three islands at 17.2m. September's 21.9m is remarkable — the only month across all three islands exceeding 20m. The autumn (September–November) average of 19.3m rivals typical Okinawa summer conditions. The deep, enveloping blue of 'Hachijo Blue' reaches its peak in autumn.

However, the seasonal range of 8.3m is the largest of the three islands, with July dipping to 13.6m. The rainy season and typhoon period likely bring coastal runoff and rain-induced turbidity. The contrast between May (19.8m) and July (13.6m) is striking — June and July are best avoided. Note that Hachijojima data covers only 2025–2026 (326 observations), and more data is needed to confirm long-term trends.

Seasonal Summary

Monthly Water Temperature (Reference)

Miyakejima water temp highlighted in red (highest of the three)

Which Island Should You Choose?

Summary

Although all three belong to the same Izu Islands chain near Tokyo, the diving experience differs dramatically. For clarity, choose Hachijojima (September is unbeatable). For stability, choose Oshima (seasonal range just 1.8m). For marine life diversity, choose Miyakejima (tropical water temperatures support extraordinary species diversity). Only by visiting all three can you truly say you know the Izu Islands.

Data source: scraped from public dive logs. Oshima: 1,309 obs; Miyakejima: 936 obs; Hachijojima: 326 obs (mainly 2025–2026).