West Izu Diving Comparison — Kumomi, Koganezaki, Osezaki, Futone & Shirasaki

West Izu (Nishi-Izu) refers to the diving region along the western coast of the Izu Peninsula, facing Suruga Bay. Known for its calm sea conditions and diverse underwater topography, it is one of the most popular diving areas for divers based in the Tokyo metropolitan region. However, lumping all West Izu sites together obscures dramatic differences in visibility patterns and AI prediction accuracy between individual sites.

This article provides a cross-site comparison of five representative West Izu dive sites: Kumomi, Koganezaki, Osezaki Bay, Futone, and Shirasaki. Drawing on a combined 5,474 days of measured visibility data, we reveal each site's seasonal patterns, explain the striking differences in prediction accuracy, and offer practical guidance on which site to choose under different conditions. The range from Osezaki Bay being difficult for AI to predict (completely unpredictable) to Shirasaki's AI accuracy 61% (highly predictable) is remarkable.

Five Sites at a Glance

• Kumomi: 1,980 observations. Famous for cave diving. AI accuracy 47%.
• Koganezaki: 1,088 observations. Premier beach dive site, rich in macro life.
• Osezaki Bay: 936 observations. Japan's most popular training site, but difficult for AI to predict — prediction is essentially impossible.
• Futone: 934 observations. Offshore boat point with pelagic encounters. AI accuracy 56%.
• Shirasaki: 536 observations. Smallest dataset, yet achieves AI accuracy 61% — the highest in West Izu.

Monthly Visibility Patterns by Site

Let us examine how visibility varies across the seasons at each site. Despite all facing Suruga Bay, each site shows distinctly different seasonal characteristics.

Kumomi — The Cave Diving Capital

Kumomi displays the classic Izu pattern of winter-high, summer-low visibility. January and February average around 14 m, when light streaming through the caves is at its most dramatic. In contrast, July drops to about 8 m as summer rains and plankton bloomsreduce water clarity. The annual range of approximately 6 m makes seasonal timing an important consideration.

Osezaki Bay — The Unpredictable

Osezaki Bay's monthly data reveals some seasonal structure, but as we discuss below, our AI model is completely unable to predict its day-to-day variations. This makes it the most unpredictable site in our entire database of 30+ dive sites nationwide, where AI prediction is essentially impossible.

Futone — Open-Ocean Dynamics

As an offshore boat point, Futone shows more dynamic visibility swings than sheltered bay sites. It receives direct influence from the Kuroshio Current, and when conditions align, visibility can exceed 20 m — exceptional by West Izu standards. Winter brings the best clarity alongside opportunities for pelagic encounters.

Cross-Site Comparison: Best Visibility by Season

Looking across all five sites, Futone consistently leads in winter (December through February) thanks to its direct exposure to clear Kuroshio water. Kumomi also performs well in winter but does not quite match Futone's peak values. Koganezaki and Osezaki Bay run at more modest levels year-round, though they compensate with calmer, more diveable conditions.

In summer, visibility drops across all sites, but the gradient of recovery is telling: ocean-facing Futone recovers fastest, while the enclosed Osezaki Bay recovers slowest. This directly reflects each site's degree of access to fresh oceanic water — a fundamental geographic factor that no amount of AI modeling can overcome.

The Osezaki Mystery — Difficult for AI to Predict

The most striking finding in this analysis is that Osezaki Bay is difficult for AI to predict. This means our AI model, armed with 45 types of weather, marine, and satellite data, explains literally none of the variation in Osezaki Bay's visibility. The model's predictions are no better than simply guessing the long-term average every day.

This is not a data scarcity issue — 936 observations is a substantial dataset. Rather, it strongly suggests that the mechanisms governing Osezaki Bay's visibility cannot be captured by conventional meteorological and oceanographic variables. Several factors likely contribute:

• Extreme topographic shelter: Osezaki Bay is enclosed by a headland that blocks open-ocean swells and currents. The bay's internal circulation patterns dominate visibility, and these cannot be captured by regional weather or marine data.
• Sediment disturbance: The bay's silty-sand bottom is easily disturbed by diver activity (especially fin kicks during training courses) and weak bottom currents, creating localized turbidity events that no satellite or weather model can detect.
• Freshwater springs: Submarine freshwater seeps exist within the bay, and the mixing of fresh and salt water creates unpredictable localized visibility changes that vary day to day.
• Suruga Bay deep-water influence: Suruga Bay reaches 2,500 m depth, making it Japan's deepest bay. Intermittent deep-water upwelling can cause unpredictable fluctuations in coastal water quality.

The Shirasaki Surprise — AI Accuracy 61% from Only 536 Observations

In stark contrast to Osezaki Bay, Shirasaki achieves AI accuracy 61% with only 536 data points — the highest prediction accuracy among all five West Izu sites. This is remarkable because AI models generally perform better with more data, yet Shirasaki outperforms Kumomi (1,980 observations, AI accuracy 47%) and Futone (934 observations, AI accuracy 56%) despite having the smallest dataset.

This suggests that Shirasaki's visibility responds in a relatively "well-behaved" manner to changes in weather and ocean conditions. The relationship between input data (wind, waves, currents, satellite chlorophyll) and visibility is more straightforward and consistent than at other sites. An exposed coastal position with stable surrounding current patterns likely contributes to this predictability. For divers, this means that weather forecasts and marine condition reports are genuinely useful for planning a trip to Shirasaki.

Futone's Strong Predictability (AI Accuracy 56%)

Futone ranks second in prediction accuracy among the five sites, with AI accuracy 56%. As an offshore boat point, Futone is directly influenced by large-scale oceanographic conditions rather than localized bay dynamics. Wave height, swell period and direction, and Kuroshio Current proximity all show a strong relationship with visibility here.

For divers, this means that weather and marine forecasts are genuinely useful for planning a Futone dive. Checking conditions before departure can meaningfully improve your chances of encountering excellent visibility — something that simply is not possible at Osezaki Bay. The contrast between these two sites, separated by mere tens of kilometers, powerfully illustrates how site topology determines predictability.

Yearly Trends

Examining year-by-year averages helps identify any long-term environmental changes across West Izu.

None of the five sites show a significant long-term decline in visibility. Year-to-year fluctuations are primarily driven by the Kuroshio Current path (straight vs. large meander configurations) and typhoon frequency. The ongoing large meander since 2017 may have slightly affected some sites, but overall the West Izu marine environment appears stable — encouraging news for the diving community.

Practical Guide: Which Site to Choose

Strong northwest winds → Kumomi

West Izu is exposed to prevailing northwest winds in winter, but Kumomi's dive points concentrate on the sheltered south side of Ushitsuki-iwa rock. It often remains diveable when other sites are closed, making it a reliable winter fallback.

Beginners and training → Osezaki Bay or Koganezaki

Osezaki Bay is one of Japan's most popular dive training sites, with calm, shallow water and easy access from Tokyo. Koganezaki offers similar beginner-friendly conditions with a sandy beach entry. While visibility is unpredictable at Osezaki (difficult for AI to predict), this matters less when the focus is skill development rather than sightseeing.

Pelagics and high visibility → Futone

The offshore boat point at Futone offers the best chance of encountering large pelagic fish and the highest visibility potential in West Izu. With AI accuracy 56%, advance condition checking via our forecasts is genuinely useful. Experienced divers seeking West Izu's best should prioritize Futone when forecasts look favorable.

Reliable, predictable conditions → Shirasaki

With the highest prediction accuracy in West Izu (AI accuracy 61%), Shirasaki is the best choice for divers who want to plan around visibility conditions. The model's strong performance means that forecast-based trip planning is most effective here among all West Izu sites.

Summary

Comparing five West Izu dive sites reveals that "West Izu" is far from monolithic. The spread from Osezaki Bay being difficult for AI to predict (completely unpredictable) to Shirasaki's AI accuracy 61% (highly predictable) is dramatic, and reflects fundamental differences in how each site interacts with its marine environment.

This diversity is itself a strength. Kumomi for caves, Koganezaki for macro, Osezaki for training, Futone for pelagics, Shirasaki for consistency — by matching the site to your objectives and conditions, you can unlock the full potential of West Izu diving.