What Will the Water Temperature Be Next Week? A Method Tested Against 65,411 Real Measurements

"Will a 5mm be enough next week — or do I need a drysuit?" This article doesn't answer with a forecast. It answers with 20 years and 36,714 measured water temperatures from dive sites across Japan, used to test a simple question: how well can you predict next week's water without any forecast at all?

The answer: last week's measurement nearly is next week's temperature

We pulled every pair of observations at the same site taken 6–8 days apart — 65,411 pairs — and compared them.

• 73% agree within ±1°C
• 91% agree within ±2°C
• Swings of 4°C or more happen just 2% of the time

So the most reliable prediction of next week's water temperature is simply the latest measured temperature at your site. Since suit choice only needs ±2°C precision, this alone gets you there nine times out of ten. Recent measurements for every site are in our historical archive.

Three ways to read the water before you travel

The catch: midsummer is when the ocean breaks the rules

Here's the finding that surprised us. Conventional wisdom (including an older version of this article) says temperatures are hardest to predict during seasonal transitions. The data says otherwise. The share of week-apart pairs differing by more than 2°C, by month:

• February: 2.3% — the most stable water of the year
• May: 8.0%, October: 6.5% — the supposed "transition chaos" is actually mild
• July: 15.5%, August: 16.8% — one week in six jumps by more than 2°C

In summer, a warm surface layer sits over cold water (a thermocline); when upwelling or a Kuroshio Current meander disturbs it, the temperature at diving depth jumps. So in July and August, don't lock in a 3mm just because last week read 26°C — check the spread of recent readings in the archive, and when in doubt, go thicker (the reasoning is in our wetsuit thickness guide).

Suit selection by water temperature

Prefer to just enter a destination and month? Use our wetsuit thickness tool.

Monthly measured averages at major sites

Two things worth noticing in the chart:

1. The ocean runs about a month behind the land. At most sites the warmest month is September, not August (Futo 24.9°C, Kerama 28.1°C). September buys you summer water with thinner crowds; conversely, March air feels like spring while the water sits at its floor (Echizen 11.4°C, Futo 15.4°C).
2. Annual range differs wildly by region. Kerama in Okinawa moves only 6.9°C all year (21.2–28.1°C — wetsuit territory even in winter), while Echizen on the Sea of Japan swings 16.0°C (11.4–27.4°C — drysuit country, and many shops simply close).

Your pre-trip routine

1. One week out: check your site's latest measured temperature in the archive — that's your prediction. Note whether it sits near the monthly mean
2. Pick your suit: use the wetsuit thickness tool or the table above. In July–August, dress one step warmer or bring layers
3. Day before: check the local shop's blog or social media — the only way to catch the 2% of sudden swings

Note: These figures describe past measured distributions, not guarantees. Cold water is a documented factor in diving incidents (DAN Japan). Local shop advice and your own comfort always come first.

Data sources

• Water temperatures: this site's database (2006–2026, 91 sites, 36,714 readings, browsable in the archive); the 65,411 week-apart pairs and 1,899 site-month climatology stats are computed from the same DB
• Sea-surface temperature: JMA daily SST / terminology: JAMSTEC ocean glossary / hypothermia: DAN Japan