Why Akinohama Stays Consistent Year-Round: Just 1.9m Monthly Variation
2026-03-16
Akinohama on Izu Oshima island is Japan's most stable major dive site, demonstrated by 1,309 real observations. Monthly average visibility ranges just 1.7m — from 13.4m (May) to 15.1m (January/December). While mainland Izu Peninsula fluctuates dramatically with spring bloom and typhoons, Akinohama maintains remarkably consistent conditions year-round. The secret lies in the buffering effect of its island location.
1.7m
Monthly range
14.3m
Annual avg
15.1m
Peak (Jan/Dec)
1,309 obs
Real observations
Akinohama Monthly Visibility Data
| Month | Vis (m) | vs Avg |
|---|---|---|
| Jan | 15.1m | +0.8m |
| Feb | 14.9m | +0.6m |
| Mar | 14.1m | -0.2m |
| Apr | 13.6m | -0.7m |
| May | 13.4m | -0.9m |
| Jun | 14.1m | -0.2m |
| Jul | 13.8m | -0.5m |
| Aug | 13.8m | -0.5m |
| Sep | 14.9m | +0.6m |
| Oct | 14.9m | +0.6m |
| Nov | 14.5m | +0.2m |
| Dec | 15.1m | +0.8m |
Stability Comparison with Other Sites
| Site | Monthly Range | Min Month | Max Month |
|---|---|---|---|
| Akinohama (Oshima) | 1.7m | 13.4m | 15.1m |
| Futo | 6.5m | 7.5m | 14m |
| IOP | 7.2m | 7.8m | 15m |
| Kushimoto | 5.8m | 8m | 13.8m |
| Tajiri | 9.1m | 6.4m | 15.5m |
Akinohama's 1.7m range is less than one-fifth of Tajiri's 9.1m. It's overwhelmingly more stable than mainland sites like Futo (6.5m) and IOP (7.2m).
Why Akinohama Is So Stable: The Island Effect
Reason 1: Minimal river water influence
Mainland coastal sites suffer major visibility drops when river freshwater and sediment flow in after rain. Izu Oshima, being an island, has no major rivers, limiting rainfall impact. The absence of mainland spring snowmelt and heavy rainy season river input is the primary stability factor.
Reason 2: Steady oceanic water supply
Akinohama sits on Izu Oshima's east coast, where Kuroshio branch currents steadily supply clear oceanic water. While the Izu Peninsula sees complex mixing of Sagami Bay coastal water and oceanic water, Oshima is surrounded by open ocean on all sides, maintaining more stable water quality.
Reason 3: Muted spring bloom impact
Mainland Izu Peninsula drops below 8m visibility from March to May due to spring phytoplankton bloom. Akinohama's lowest is 13.4m in May — far less dramatic. The island's surrounding oceanic water influence is stronger, with less coastal nutrient supply from rivers.
Akinohama's Appeal: Reliable Conditions Any Time of Year
Great for beginners
With visibility always above 13m, beginners can dive comfortably. It has the lowest risk of arriving to find murky conditions.
Hidden gem in spring
When mainland Izu suffers from spring bloom, Akinohama maintains 13-14m. A short trip extension gives you comfortable diving.
Winter is best
January and December peak at 15.1m. Winter Izu Oshima has fewer crowds on ferries and jet boats for a relaxed experience.
Access
About 1hr 45min by jet boat from Tokyo Takeshiba, or 6 hours by ferry. 25 minutes by Cessna from Chofu Airport. Day trips possible.
'Stability' vs 'Peak Clarity': Akinohama vs Hachijojima
Within the Izu Islands, Hachijojima averages a higher 17.1m but swings widely (13.6m in Jul to 21.9m in Sep, 8.3m range). Akinohama averages a lower 14.3m but barely varies (1.7m range). Choose Akinohama for 'reliably good conditions' and Hachijojima for 'chasing the best possible day.'
About the Data
Akinohama data collected from CSV data and dive shop reports (1,309 observations). Monthly averages use all observations per month. Site comparisons use full historical data. Monthly range calculated as highest month average minus lowest month average.
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