Early Summer Diving in Japan (May): Nationwide Visibility & Water Temp Rankings
2026-03-11
March 11, 2026 · 14 dive sites analyzed from real observation data
May Ocean: Where 'Spring Bloom' Meets 'Rising Water Temp'
May is the month in Japan's diving calendar where site selection matters most. Okinawa is oblivious to spring bloom, delivering 24m+ clarity in 26°C oceanic water at its finest. Meanwhile, Izu Peninsula wallows in spring bloom's peak at 9–10m. The same 'May in Japan' sees a 14m+ visibility gap between regions. We analyze real data from 14 sites to identify May's best diving spots.
May Visibility Rankings (14 Sites)
| Rank | Site | Region | May Vis. | May Temp. | Note |
|---|---|---|---|---|---|
| 1 | Yonaguni | Okinawa | 24.2m | 26.3°C | Peak season before rainy season; Kuroshio clarity |
| 2 | Ishigaki | Okinawa | 20.1m | 26°C | Sekisei Lagoon coral active; 26°C tropical fish |
| 3 | Kerama | Okinawa | 19.4m | 23.8°C | Day-trip from Naha; sea turtle nesting begins |
| 4 | Amami Oshima | Kagoshima | 16.9m | 22.4°C | Fewer tourists; comfortable 22°C water |
| 5 | Ito | Chiba | 15.9m | 16.9°C | Near Tokyo; May is annual best; mild spring bloom |
| 6 | Shirahama | Wakayama | 14.9m | 19°C | Visibility rises as Kuroshio approaches in spring |
| 7 | Akinohama (Oshima) | Tokyo | 13.4m | 17.5°C | Oshima; one of Japan's most stable sites year-round |
| 8 | Sado | Niigata | 13.1m | 15°C | Sea of Japan; still good in May; cold 15°C water |
| 9 | Kashiwajima | Kochi | 11.5m | 20.1°C | Mild spring bloom; rare species still abundant |
| 10 | Kushimoto | Wakayama | 10.8m | 19.7°C | Slight improvement from April; holds 10m+ before rainy season |
| 11 | IOP | Shizuoka | 10.6m | 17.8°C | Deep in spring bloom; down from 18.6m in winter |
| 12 | Mikomoto | Shizuoka | 9.7m | 18.1°C | Spring bloom peak; improves from June |
| 13 | Futo | Shizuoka | 9.2m | 17.7°C | Nudibranch season; low clarity but high marine life |
| 14 | Echizen | Fukui | 7.7m | 16.3°C | Sea of Japan worst in May; wait until August |
Choosing by Water Temperature in May
| Temp. Zone | Representative Sites | Recommended Suit |
|---|---|---|
| 25°C+ (Okinawa) | Yonaguni (26.3°C), Ishigaki (26.0°C), Kerama (23.8°C) | 3mm wetsuit |
| 20–24°C (Kuroshio zone) | Amami (22.4°C), Shirahama (19°C), Miyakejima (21.6°C), Kashiwajima (20°C) | 5mm wetsuit |
| 15–19°C (Izu/Chiba) | Ito (16.9°C), IOP (17.8°C), Futo (17.7°C), Mikomoto (18.1°C) | 5–7mm wetsuit |
| ≤15°C (Sea of Japan) | Sado (15°C), Echizen (16.3°C), Tajiri (16.4°C) | Drysuit recommended |
Key Points for Reading May Ocean Conditions
Okinawa in 'Golden Pre-Rainy Season'
Okinawa's rainy season typically begins in late May to early June. This means the Golden Week period in early May is the 'golden pre-rainy season,' with Kuroshio oceanic water steadily supplied and temperatures reaching 26°C. Yonaguni (24.2m), Ishigaki (20.1m), and Kerama (19.4m) all show top-tier annual visibility. Targeting Okinawa during this period is undoubtedly the right choice.
Izu: Embrace the Spring Bloom Experience
Visibility alone makes May Izu look poor (IOP 10.6m, Futo 9.2m). However, spring bloom and marine life richness are two sides of the same coin. Nudibranchs that feed on plankton peak in species and individual counts in May. At Futo, Osezaki, and Kumomi, over 100 nudibranch species have been recorded. For those who prioritize marine life over clarity, May in Izu is the best season.
Ito (Chiba): The Kanto-Region Hidden Gem
Ito in Chiba records 15.9m in May — exceptional visibility for a near-Tokyo site. May is actually its annual clarity peak. Oceanic water from Sagami Bay flows in easily, limiting spring bloom effects. For day-trip diving from the Kanto region in May, choosing Ito over Izu is the smarter data-driven decision.
May Diving by Goal
Visibility & Warmth
→ Yonaguni, Ishigaki, Kerama
24m+, 26°C world-class conditions. GW Okinawa trip is the top choice
Near Tokyo
→ Ito (Chiba)
Annual best 15.9m in May. Best clarity within 3h of Tokyo
Nudibranch & Marine Life
→ Futo, Kumomi, Osezaki
In spring bloom but nudibranch species and counts peak. Dense marine life even at 9m visibility
Summary
May diving in Japan comes down to: Okinawa for visibility, Izu for marine life. For clarity, Okinawa (Yonaguni, Kerama, Ishigaki) is unmatched at 25°C+ water. For rich marine life despite low clarity, Izu is the best. For efficient diving near Tokyo, data clearly points to Ito (Chiba). The most important rule for May planning: decide your goal first, then choose your site.
Data source: scraped from public dive logs. May monthly averages from real observations.
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