Sea temperature and swimming season in Naha

The sea off Naha is comfortable for swimming all year round — it never drops below 21.8°C, and peaks at 29.2°C in August.

"Warm enough" is a judgement, not a fact

There is no objective temperature at which the sea becomes swimmable. A Norwegian and a Spaniard will disagree, and neither is wrong. The season above uses 19.0°C — the bottom of the range most people describe as comfortable in swimwear — but here is what other thresholds imply:

If you are	Minimum	Months you can swim
Hardy	16.0°C	12 of 12
Most people	19.0°C	12 of 12
Prefer it warm	22.0°C	11 of 12
Prefer it hot	25.0°C	6 of 12

Month by month

Sea temperature, the air you climb out into, and the wind that decides how that feels.

Sea temperature, air temperature, wind and daylight by month.
Month	Sea	Air	Wind	Feels	What to wear
January	22.6°C	18.8°C ↓	7.9 m/s	Very comfortable	none, or a rash vest
February	21.8°C	18.5°C ↓	7.8 m/s	Comfortable	none, or a rash vest
March	22.2°C	19.9°C ↓	6.4 m/s	Very comfortable	none, or a rash vest
April	23.2°C	22.2°C ↓	6.1 m/s	Very comfortable	none, or a rash vest
May	24.9°C	24.5°C ↓	6.3 m/s	Very comfortable	none needed
June	27.0°C	27.8°C	7.5 m/s	Bath-like	none needed
July	29.0°C	29.0°C ↓	6.5 m/s	Bath-like	none needed
August	29.2°C	29.3°C	7.6 m/s	Bath-like	none needed
September	29.2°C	29.1°C ↓	5.6 m/s	Bath-like	none needed
October	28.3°C	26.0°C ↓	10.4 m/s	Bath-like	none needed
November	26.2°C	23.7°C ↓	6.4 m/s	Bath-like	none needed
December	24.2°C	19.6°C ↓	7.4 m/s	Very comfortable	none needed

Air is the average daily maximum — roughly what you would meet swimming in the middle of the day. An arrow marks months where the air is colder than the sea.

When in the day can you swim?

Daylight and burn risk by month, in Asia/Tokyo local time.

Month	Sunrise	Sunset	Daylight	Peak UV
January	07:18	17:58	10h 40m	5.2
February	07:05	18:21	11h 17m	7.1
March	06:38	18:37	11h 59m	9.4
April	06:06	18:52	12h 46m	11.4
May	05:42	19:08	13h 25m	12.3
June	05:36	19:22	13h 46m	12.7
July	05:46	19:24	13h 38m	12.9
August	06:01	19:06	13h 04m	12.7
September	06:14	18:34	12h 19m	11.2
October	06:28	18:02	11h 34m	8.7
November	06:47	17:39	10h 52m	6.1
December	07:09	17:39	10h 31m	4.8

Open water burns faster than a beach: the surface reflects sunlight up at you, and sunscreen washes off in the first few minutes. A long swim under a UV index above 6 needs a rash vest more than it needs more cream.

What the bands mean

From	Description	In practice
above 26°C	Bath-like	Warm enough that it offers no cooling at all
above 22°C	Very comfortable	Swimwear, no time limit for most people
above 19°C	Comfortable	Pleasant once you are in; most people swim happily
above 16°C	Bracing	Swimmable but short; many prefer a shorty wetsuit
above 12°C	Cold	Wetsuit territory. Unprotected swims should be brief
below 12°C	Dangerous	Cold shock risk. Specialist gear and supervision

Do you need a wetsuit in Naha?

Some months, yes. The sea drops to 21.8°C in February. What thickness, month by month, for swimming and for surfing →

Other coastal cities in Japan

Where these numbers come from, and what they miss

Open water, not your beach. The measurement is a satellite cell about 28 km across, centred 11.1 km from Naha. Shallow bays and lagoons run warmer; exposed coasts with upwelling run colder, sometimes by several degrees.
Surface temperature. Below a few metres the sea is often noticeably colder, and in summer a warm surface layer can sit over much colder water.
A six-year average, not a thirty-year normal. Sea temperature varies little between years, so this is close — but it is not a climate normal, and any individual year will differ.
Averages hide storms. A monthly mean says nothing about the week a cold current arrives.

Full methodology, sources and known limitations