Koganezaki vs IOP: West Izu vs East Izu Visibility Showdown
2026-03-16
Koganezaki and Izu Oceanic Park (IOP) represent the west and east coasts of the Izu Peninsula. Their annual average visibility is nearly identical — roughly 13.5m and 13.8m respectively. But their seasonal patterns tell a very different story. We analyzed real dive log data to reveal each site's strengths.
~13.5m
Koganezaki avg
~13.8m
IOP avg
8.5m
IOP seasonal range
6.5m
Koganezaki range
Monthly Visibility Comparison
| Month | Koganezaki(m) | IOP(m) | Diff |
|---|---|---|---|
| Jan | 13.2m | 18.6m | IOP +5.4 |
| Feb | 14.5m | 17.8m | IOP +3.3 |
| Mar | 15.8m | 14.2m | Kog +1.6 |
| Apr | 14.1m | 12.5m | Kog +1.6 |
| May | 12.8m | 10.1m | Kog +2.7 |
| Jun | 11.5m | 10.3m | Kog +1.2 |
| Jul | 10.8m | 10.5m | Kog +0.3 |
| Aug | 11.2m | 11m | Kog +0.2 |
| Sep | 12.5m | 12.8m | IOP +0.3 |
| Oct | 14m | 14.5m | IOP +0.5 |
| Nov | 15.2m | 16.8m | IOP +1.6 |
| Dec | 15m | 17.5m | IOP +2.5 |
Seasonal Pattern Differences
Winter (Dec-Feb): IOP Dominates
IOP peaks at 18.6m in January, exceeding Koganezaki by 3-5m. East Izu benefits from winter northwest winds blowing offshore, calming the sea and allowing clear Kuroshio water to flow in. IOP's true strength shines in winter.
Spring (Mar-Apr): Koganezaki Takes the Lead
Koganezaki peaks at 15.8m in March, beating IOP by 1.6m. While IOP suffers from spring plankton bloom, Koganezaki on the west coast maintains relatively clear water. For spring diving in Izu, Koganezaki has the edge.
Summer (Jul-Aug): Both Sites Drop
Both sites drop to the 10-11m range in summer with minimal difference. Rising water temperatures promote plankton growth, reducing visibility across all of Izu.
Practical Comparison
| Factor | Koganezaki | IOP |
|---|---|---|
| Annual avg | ~13.5m | ~13.8m |
| Peak month | Mar (15.8m) | Jan (18.6m) |
| Seasonal range | 6.5m | 8.5m |
| Stability | Higher | More variable |
| Crowding | Less crowded | More crowded |
| Spring bloom | Less affected | More affected |
When to Choose Which
Choose IOP When
- Targeting peak winter visibility (Dec-Feb)
- Want to experience 18m+ clear water
- Enjoy dynamic underwater topography
Choose Koganezaki When
- Diving in Izu during spring (Mar-Apr)
- Prioritizing stable, predictable visibility
- Prefer less crowded, relaxed diving
About the Data
Analysis based on real dive log data collected from dive shop daily reports at both sites. IOP has 3,151 observations and Koganezaki also has substantial data. Visibility uses daily average values.
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