Futo vs Hirasawa: Why Neighboring Izu Sites Differ by 2.7m
2026-03-16
Futo and Hirasawa are popular dive sites on Izu's east coast, relatively close to each other. Yet their annual average visibility differs by 2.7m — Futo at 11.5m (3,495 obs) vs Hirasawa at 8.8m (2,765 obs). Why such a gap between neighbors? We use data to explain the geography and current dynamics behind this difference.
11.5m
Futo avg
8.8m
Hirasawa avg
2.7m
Avg difference
6,260 obs
Total observations
Monthly Visibility Comparison
| Month | Futo(m) | Hirasawa(m) | Diff |
|---|---|---|---|
| Jan | 15.6m | 11.5m | +4.1m |
| Feb | 14.8m | 10.8m | +4.0m |
| Mar | 12.5m | 9.5m | +3.0m |
| Apr | 10.8m | 8m | +2.8m |
| May | 9.2m | 7.2m | +2.0m |
| Jun | 8.5m | 6.8m | +1.7m |
| Jul | 8.2m | 7m | +1.2m |
| Aug | 9m | 7.5m | +1.5m |
| Sep | 10.5m | 8.2m | +2.3m |
| Oct | 12m | 9m | +3.0m |
| Nov | 13.5m | 10.2m | +3.3m |
| Dec | 14.5m | 11m | +3.5m |
Futo exceeds Hirasawa every month. The gap is largest in winter (4.1m) and smallest in summer (1.2-1.7m).
Why the 2.7m Difference Exists
Geography: Open Ocean vs Enclosed Bay
Futo faces the open ocean, receiving clear Kuroshio-influenced water directly. Hirasawa sits inside a bay, surrounded by land, which slows water exchange. This geographic difference directly impacts visibility.
Bottom Type: Rocky vs Sandy
Hirasawa has a sandy bottom where fin kicks and landings easily stir up sediment. The calm bay environment also accumulates sediment, contributing to lower visibility. Futo's rocky bottom is less prone to turbidity from water disturbance.
Why the Gap Widens in Winter
In winter, northwest winds push clear offshore water toward open-ocean sites like Futo (Jan: 15.6m). This effect is limited for bay-enclosed Hirasawa, which plateaus at 11.5m. The gap reaches 4.1m. In summer, plankton growth reduces both sites equally, narrowing the gap.
Hirasawa's Strengths: Visibility Isn't Everything
Calm Conditions
Protected bay means small waves, ideal for beginners and rusty divers. Hirasawa may stay open on rough days when Futo closes.
Easy Beach Entry
Gentle sandy beach makes entry and exit easy. Perfect environment for training and refresher dives.
Rich Macro Life
Sandy environment hosts unique marine life. Popular with macro photographers for nudibranchs, gobies, and seahorses.
When to Choose Which
Choose Futo When
- Want 15m+ winter clarity
- Want dynamic rocky topography
- Intermediate+ divers
Choose Hirasawa When
- Beginners, training, refresher dives
- When open-ocean sites close in bad weather
- Macro photography
About the Data
Analysis based on real dive log data from Futo (3,495 obs) and Hirasawa (2,765 obs). The combined 6,260 observations represent the richest dataset in the Izu area, enabling highly reliable comparison. Visibility uses daily average values.
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