Days When Visibility Exceeded 30m: Japan's All-Time Clearest Dives — 1,840 Records

Analyzing 30m+ Visibility: 1,840 Records from 46,000 Dives

Every diver dreams of 30-meter visibility. Our database of 46,000+ real dive log observations across 24 sites in Japan contains 1,840 records of visibility at or above 30m (capped at realistic ≤55m to exclude anomalies). Which sites, which months, and how often?

Site Rankings: 30m+ Record Counts

The data shows extreme concentration at specific sites.

Yonaguni: 30m+ Is Almost Normal

Yonaguni (Okinawa) accounts for 81.5% (1,500 records) of all 30m+ sightings. Its hit rate of 31.1% means one in three dives there exceeds 30m visibility.

Yonaguni: Monthly 30m+ Hit Rates

September peaks at 51.7% — more than one in two dives exceeds 30m. July through October all exceed 40%. Even in January, the rate holds at 14.9%.

Best Strategies for 30m+ Visibility

Surest bet: Yonaguni in September–October

Near-50% hit rates make September/October Yonaguni the statistically strongest combination. January–March (hammerhead season) still holds 15–20% — combining big animals with clear water.

Best on Honshu: Ito (Chiba) in December–January

91 records and a 4.6% hit rate make Ito the Honshu champion. Just 1.5 hours from Tokyo, winter months average 17–18m with occasional 40m days after calm spells.

Summer surprise: Yakushima in July–August

A 41.4% hit rate from limited data (157 obs). July average of 32.1m exceeds Yonaguni's 25.6m. Worth pairing with the UNESCO World Heritage forest.

Key Takeaways

• 81.5% of all 30m+ records are from Yonaguni (1,840 total)
• Yonaguni September: 1 in 2 dives exceeds 30m (51.7%)
• Honshu leader: Ito (Chiba) with 91 records / 4.6% hit rate
• Peak months globally: August (234), September (230), July (215)
• Yakushima shows 41.4% from small sample — a compelling outlier