Hirasawa Visibility Analysis: What 2,600 Days of Data Reveal About This Beginner-Friendly Dive Site

Often dismissed for low visibility, Hirasawa averages 8.8m. But there's a reason it's Japan's #1 beginner training site — and the answer isn't about clarity. 2,694 days of data reveal its true value.

The extensive dataset accumulated from 2017 to 2026 reveals the true picture of Hirasawa's visibility. The conclusion: Hirasawa is far more than a "low-visibility beginner spot" — in winter, it delivers genuinely clear water that is well worth experiencing.

Monthly Visibility: Winter Clear Water Deserves Attention

Hirasawa's annual average visibility is 8.8m. Compared to Izu Oceanic Park's annual average of roughly 14m, this is somewhat lower, but it is a reasonable figure given the characteristics of a sheltered bay site. On a monthly basis, the highest visibility is 11.5m in January, while the lowest is 7.0m in July.

This pattern aligns with the "clear winters, murky summers" trend common across the entire Izu region. An 11.5m reading in January is quite impressive for a beginner-friendly site, and with a drysuit you can enjoy a comfortable underwater stroll through crystal-clear winter water.

Why Is Hirasawa's Visibility on the Lower Side?

The reasons for Hirasawa's lower visibility compared to open-ocean sites come down to geography. Situated inside a bay, it does not benefit from the same level of tidal flushing as exposed sites. The predominantly sandy bottom is also prone to silt disturbance from swells and diver activity. Freshwater inflow from nearby rivers further contributes to reduced clarity.

However, these very characteristics are the flip side of what makes Hirasawa "calm and safe." The absence of strong currents and heavy swells — unlike open-ocean sites — is precisely what maintains a secure environment for beginners.

Yearly Trends: A Stable Track Record

Looking at the year-over-year trends from 2017 to 2026, Hirasawa's visibility has remained relatively stable. No significant downward trend is apparent, and year-to-year variation is limited. This suggests that the sheltered bay is less susceptible to large-scale oceanic fluctuations such as Kuroshio Current meandering. This stability also means that conditions are easier to predict when planning a visit.

Getting the Most Out of Hirasawa

Here are our data-backed tips for enjoying Hirasawa. If visibility is your priority, December through February is the best window. You can expect averages in the 10 to 11m range, and winter-exclusive macro subjects like lumpfish (Dango-uo) are plentiful. In summer, visibility drops to around 7m, but the warm water makes for comfortable diving — ideal for training dives and refresher courses.

Hirasawa also features artificial objects and fish reefs placed underwater, providing plenty of points of interest even on days when visibility is modest. For macro photography enthusiasts, subject availability matters more than visibility, and Hirasawa is a treasure trove of nudibranchs and crustaceans.

Summary

Our analysis of 2,696 days of Hirasawa data reveals the following. The annual average visibility is 8.8m, peaking in January (11.5m) and bottoming out in July (7.0m). The site follows the Izu region's typical winter-clear pattern, offering genuinely enjoyable visibility during the colder months.

Yearly trends are stable, making conditions highly predictable. Don't write off Hirasawa based solely on its beginner-friendly reputation — the winter clear water here is an experience worth seeking out.