Osezaki Bay vs Open Ocean: Why Visibility Differs by 3m at the Same Dive Site
2026-03-15
Osezaki on Izu's west coast is one of the closest dive sites for Tokyo-area divers. But did you know visibility differs dramatically between its 'Bay,' 'Open Ocean,' and 'Point' areas — even on the same day? From 3,381 observations, we found annual averages of Bay 7.6m vs Open Ocean 10.6m vs Point 10.5m — a consistent 3m gap.
7.6m
Bay (2,016 obs)
10.6m
Open Ocean (931)
10.5m
Point (434 obs)
Data period: June 2019 – January 2024
Monthly Visibility Comparison: Three Sites Side by Side
| Month | Bay | Open | Point | Diff |
|---|---|---|---|---|
| Jan | 11.8m | 12.8m | 13.2m | +1m |
| Feb | 10.7m | 12.6m | 13.6m | +1.9m |
| Mar | 8.6m | 11.1m | 10.5m | +2.5m |
| Apr | 6.6m | 9m | 9.2m | +2.4m |
| May | 7.8m | 10.4m | 12.3m | +2.6m |
| Jun | 7.5m | 11.5m | 11m | +4m |
| Jul | 6.5m | 8.7m | 8.7m | +2.2m |
| Aug | 5.9m | 10.3m | 8.9m | +4.4m |
| Sep | 6.5m | 10.9m | 8.8m | +4.4m |
| Oct | 6.5m | 9.7m | 10.6m | +3.2m |
| Nov | 7.8m | 10m | 9.9m | +2.2m |
| Dec | 9.5m | 12.8m | 13.3m | +3.3m |
Finding: Largest Gap in Summer (August) at +4.4m
August: Bay 5.9m vs Open Ocean 10.3m — a 4.4m gap. Summer degrades the bay badly while the open ocean stays relatively clear. In winter (Dec–Feb), the gap narrows and the bay occasionally exceeds 10m.
Why a 3m Difference at the Same Location?
Bay: Enclosed Geography Traps Turbidity
Osezaki Bay is semi-enclosed, surrounded by land on three sides. Its sandy-muddy bottom easily suspends sediment from fin kicks and wave action. Freshwater inflow from rivers and groundwater also contributes. Located deep in Suruga Bay, water exchange with the open ocean is slow — once turbid, it takes time to clear.
Open Ocean: Direct Oceanic Water Influence
The open ocean side faces Suruga Bay with good water circulation. Kuroshio-influenced oceanic water flows in more easily, bringing lower plankton concentrations and clearer water. Rocky bottom means less sediment suspension.
Point: Ocean Water + Current Convergence
The point sits at the cape's tip where two currents converge. Visibility matches the open ocean (10.5m avg), and in winter the point sometimes exceeds it (Feb: Point 13.6m > Open 12.6m). Stronger currents make it advanced-level, but large pelagic encounters increase.
Choosing Your Dive at Osezaki
| Site | Visibility | Level | Best For |
|---|---|---|---|
| Bay | 7.6m | Beginner | Training, beginners, macro photography |
| Open Ocean | 10.6m | Intermediate+ | Soft coral, wide-angle, visibility-focused |
| Point | 10.5m | Advanced | Pelagics, drift diving, winter clarity |
About the Data
3,381 observations from Osezaki dive shop daily logs (Bay 2,016, Open 931, Point 434). June 2019 – January 2024. Visibility is the mean of daily min/max. The bay has the most data because of its popularity for training and beginners.
Reference: Suruga Bay ocean conditions from Shizuoka Prefectural Fisheries and Marine Technology Research Institute.
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