Ito (Tateyama) Seasonal Deep Dive — Why Winter Peaks at 18.8m
2026-03-16
Ito in Tateyama, Chiba is famous for its shark encounters, but it also boasts top-tier mainland visibility. Its 15.9m annual average ranks #2 on Honshu (after Hachijojima 17.1m). Winter (Dec 18.7m / Jan 18.8m) slightly edges IOP's winter peak (18.6m). We analyze 1,985 observations for a complete seasonal guide.
Monthly Visibility & Water Temp
| Month | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Vis | 18.8 | 17.4 | 16.4 | 14.8 | 15.9 | 15.7 | 13.2 | 15 | 14.7 | 15.8 | 15 | 18.7 |
| Temp | 16.7℃ | 16.1℃ | 15.8℃ | 16.1℃ | 16.9℃ | 18.6℃ | 19.7℃ | 20.5℃ | 22.3℃ | 22.6℃ | 21.1℃ | 18.4℃ |
Three Seasonal Characteristics
1. Winter Visibility Exceeds IOP
Jan 18.8m and Dec 18.7m slightly beat IOP (Jan 18.6m) — the highest for any mainland peninsula site. Just 2 hours (100km) from Tokyo, making it ideal for winter day-trip diving. Water temp 16–17°C; drysuit recommended.
2. July Is the Only Real Dip
Ito stays above 14m year-round, with July (13.2m) as the only dip. Unlike IOP's April crash to 10.1m, Ito avoids spring bloom — likely because southern Boso Peninsula geography shields it from the main plankton bloom path.
3. Remarkably Stable Year-Round
Annual range of 5.6m (18.8m to 13.2m) is smaller than IOP (8.5m) or Futo (6.4m). Ito offers rare consistency — you can visit anytime and expect decent visibility.
Seasonal Recommendations
Winter (Dec–Feb): Peak Clarity
17–19m. Drysuit required. Sharks + clear water = best combo
Spring (Mar–May): Strong When Others Fade
15–16m. Holds 14.8m when IOP drops to 10m in April
Summer (Jun–Aug): For Warm Water
13–16m. Water 19–21°C, wetsuit OK. Watch July dip
Autumn (Sep–Nov): Best Balance
15–16m. Water 21–23°C. Clarity + comfort combined
About the Data
Ito (Tateyama, Chiba) 1,985 observations (2006–Mar 2026). 160+ samples per month — highly reliable.
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