Sea temperature and swimming season in Shizuoka

The sea off Shizuoka is comfortable for swimming from May to November. It peaks at 27.9°C in August and drops to 16.0°C in February.

"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	7 of 12
Prefer it warm	22.0°C	5 of 12
Prefer it hot	25.0°C	3 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	16.5°C	11.5°C ↓	2.7 m/s	Bracing	3/2 mm full suit
February	16.0°C	8.2°C ↓	2.6 m/s	Bracing	3/2 mm full suit
March	16.1°C	11.9°C ↓	2.3 m/s	Bracing	3/2 mm full suit
April	17.6°C	19.5°C	2.6 m/s	Bracing	3/2 mm full suit
May	19.3°C	20.5°C	2.4 m/s	Comfortable	shorty, or nothing if you are acclimatised
June	22.3°C	24.0°C	1.8 m/s	Very comfortable	none, or a rash vest
July	25.6°C	29.0°C	1.9 m/s	Very comfortable	none needed
August	27.9°C	28.7°C	1.6 m/s	Bath-like	none needed
September	27.4°C	27.0°C ↓	2.1 m/s	Bath-like	none needed
October	24.4°C	21.3°C ↓	2.1 m/s	Very comfortable	none needed
November	21.9°C	15.4°C ↓	2.4 m/s	Comfortable	none, or a rash vest
December	18.5°C	13.1°C ↓	2.7 m/s	Bracing	shorty, or nothing if you are acclimatised

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	06:54	16:57	10h 03m	2.9
February	06:32	17:28	10h 56m	4.4
March	05:57	17:53	11h 56m	6.4
April	05:14	18:18	13h 04m	8.5
May	04:43	18:42	13h 59m	9.8
June	04:32	19:01	14h 30m	10.5
July	04:43	19:01	14h 17m	10.6
August	05:06	18:35	13h 29m	10.1
September	05:29	17:54	12h 25m	8.3
October	05:52	17:12	11h 20m	5.8
November	06:20	16:41	10h 21m	3.6
December	06:46	16:36	9h 49m	2.6

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 Shizuoka?

Some months, yes. The sea drops to 16.0°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 12.1 km from Shizuoka. 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