Onagawa Diving Visibility & Water Temperature — Tohoku's Only Data
2026-03-16
Onagawa in Miyagi Prefecture is the only Tohoku dive site with continuous visibility records. Though just 52 observations (Nov 2025–Mar 2026), this data is invaluable as the only diving visibility record from Japan's Tohoku region. We analyze the reality of cold-water diving along the Sanriku Coast.
Key Stats
5.9m
Avg visibility
10.5℃
Avg water temp
52
Observations
7.6m
Best (Nov)
Monthly Visibility & Water Temp
| Month | Visibility | Water Temp | Obs |
|---|---|---|---|
| Nov | 7.6m | 14.6℃ | 11 |
| Dec | 5.8m | 11.7℃ | 12 |
| Jan | 3.8m | 9.3℃ | 17 |
| Feb | 6.5m | 7.3℃ | 11 |
| Mar | — | 7℃ | 1 |
Key Characteristics
Cold Water Diving Territory
Water temperature drops to 7.3°C in February, reaching only 14.6°C even in the warmest month (November). Drysuits with thick undergarments are essential. But Sanriku's cold, nutrient-rich waters host abundant sea squirts, nudibranchs, and other cold-water species.
Visibility Well Below National Average
Against a national average of 14m, Onagawa averages 5.9m — dropping to 3.8m in January. The Sanriku Coast is influenced by the nutrient-rich Oyashio (cold) Current, promoting plankton growth that reduces visibility. This is the reverse of the 'clear water paradox' — nutrient-rich waters teem with life but are murky. (JMA Tohoku Sea Conditions)
November Is the Best Season
November offers the best conditions: 14.6°C water and 7.6m visibility. Summer (Jun–Sep) data doesn't exist yet, but the Oyashio weakens in summer — potentially allowing better visibility. Future data collection will tell.
National Comparison
| Site | Avg Vis | Winter Temp | Region |
|---|---|---|---|
| Onagawa | 5.9m | 7-9℃ | Tohoku |
| Echizen | 9.7m | 10-12℃ | Sea of Japan |
| Hirasawa | 8.8m | 14-16℃ | West Izu |
| Shakotan | 11.8m | 5-8℃ | Hokkaido |
Even among cold-water sites, Hokkaido's Shakotan (11.8m) has double Onagawa's (5.9m) visibility. Shakotan benefits from the tail end of the Tsushima Warm Current. Onagawa sits in the nutrient-rich Oyashio zone, keeping visibility lower.
About the Data
52 observations from Onagawa, Miyagi (Nov 2025–Mar 2026). Winter-only data; summer visibility unknown. As data accumulates, year-round analysis will become possible.
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