Which Sites Get Clearer (or Murkier) When Water Hits 25°C+?
2026-03-16
Water above 25°C is wetsuit-comfortable diving, but visibility splits between paradise and disappointment. Yakushima hits 29.3m at 25°C+ while Hirasawa drops to 7.0m — a 4x difference in the same temperature. We analyzed all sites to reveal who wins and loses in warm water.
Sites That Clear Up in Hot Water
Sites That Get Murkier in Hot Water
Rule: Open Ocean Improves, Coastal Worsens
What Improving Sites Share
Open ocean or island sites. Water above 25°C signals Kuroshio-origin water inflow — nutrient-poor and clear. Yakushima's +11.7m reflects direct Kuroshio main stream impact.
What Worsening Sites Share
Coastal and bay sites. Their 25°C+ comes from solar heating of local water — not Kuroshio. Warming triggers phytoplankton blooms, reducing visibility. Hirasawa (-2.9m) in its enclosed bay is hit hardest; Ito (-2.5m) also warms locally along Boso Peninsula.
Practical Guide
- Summer clarity → Yakushima, Hachijojima, Kerama (warm = clear type)
- Summer Izu → Mikomoto (+4.3m improvement), open ocean advantage
- Hirasawa/Ito are best in winter → avoid hot season (visibility drops 2–3m)
- Echizen/Tajiri are summer-OK exceptions → Sea of Japan improves in warm season
About the Data
Comparing visibility on days above 25°C vs 15–20°C (moderate baseline). Sites with 100+ observations included.
🌊 Calculators for divers
Wetsuit thickness, air consumption, nitrox MOD, dive weights — all calculated on your device, so they work with no signal on the boat.
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