Futo Water Temperatures, 3,259 Measurements — Which Months Are Truly Wetsuit-Safe?
2026-03-09
"When can you dive Futo without a drysuit?" Futo, on the east coast of the Izu Peninsula near Itō, is one of the region's classic shore-diving sites. We answer with 3,259 measured water temperatures (2006–2026), and the answer fits in one line: July through November is safe; June is a coin flip.
Quick reference by month (measured)
| Period | Mean temp | What the data says | Suit |
|---|---|---|---|
| Dec–Mar | 15.1–18.1°C | In February, 2 days in 3 are below 16°C (record low 13.0°C) | Drysuit + thick undergarments |
| Apr–May | 16.2–17.7°C | Even in May, about half the days read below 18°C | Drysuit |
| June | 19.8°C | 51% chance of sub-20°C water — a genuine 50/50 | 5mm + hood if gambling; drysuit if not |
| Jul–Aug | 21.9–24.2°C | Sub-20°C odds drop to 11% in July | 5mm wetsuit |
| September | 24.9°C (yearly peak) | 87% of days at 24°C or warmer | 5mm (3mm on good days) |
| Oct–Nov | 21.0–23.5°C | November: zero readings below 18°C (of 295) | 5mm (add a hood in November) |
Want it tuned to your own cold tolerance? Use the wetsuit thickness tool.
Three things the data reveals about Futo
① The best month is September, not August. Besides the yearly-high 24.9°C average, 87% of September days deliver 24°C or warmer — clearly ahead of August's 61%. Summer crowds leave; summer water stays.
② November is safer than its reputation. Contrary to the "autumn is already cold" instinct, not one of 295 November measurements dipped below 18°C (mean 21.0°C). It's a confident final month for wetsuit divers — but December is a cliff, with 42% of days below 18°C.
③ In winter, Futo is the coldest of Izu's major sites. Its February mean of 15.1°C runs about 1°C below neighboring Izu Ocean Park, and 2 days in 3 dip under 16°C. The trade-off: winter visibility is among the year's best, and it's nudibranch and lumpfish season. Worth it — in a drysuit with proper undergarments.
Monthly temperatures vs. neighboring IOP
Futo's sheltered cove makes for easy entries, but its water runs slightly cooler than IOP's year-round — with the gap widest in winter.
Should you gamble on June?
June's 19.8°C average is often quoted as "switchover time," but averages mislead here: measured data says a June wetsuit dive hits sub-20°C water every other time (from our five-site Izu probability analysis). The right move is to check last week's measured readings in the historical archive right before you go — last week's measurement is the best predictor of next week's water, as we've verified.
Note: These are guidelines from past measured distributions. Cold-water risk is real (DAN Japan); local shop advice and your own comfort come first.
Data sources
- Water temperatures: this site's database (Futo, 3,259 readings, 2006–2026, browsable in the archive)
- Regional conditions: JMA sea-surface temperatures / Kuroshio Current
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