August · 19.1 °C

21 published records ordered by mean surface temperature in August, spanning 14.5–27 °C. The warmest is not automatically the right one for you: heated pools are flat all year while natural water follows the season.

Water temperature ranking for August
Spot	Type	Region	Max depth	Walk	January	August
Qiandao Lake, Laoshan Bay An artificial lake with 1,078 islands, holding drinking-water grade Ⅰ water — so clean that the visibility below the surface reaches 7 m in winter.	Lake	Zhejiang, China	100 m	5 min	7°	27°
Pomena, Mljet A small harbour on the wooded island of Mljet where the sea reaches 25 °C in August and the swim out to the islet of Pomestak takes 12 minutes.	Sea	Dalmatia, Croatia	22 m	1 min	13.5°	25°
Fuxian Lake, Luchong shore A deep rift lake on the Yunnan plateau with 155 m of depth and visibility over 8 m — biologically closer to Baikal than to a lowland Chinese lake.	Lake	Yunnan, China	155 m	4 min	10°	23.5°
Lake Bled, Velika Zaka A warm, small, spring-fed lake with a 6 km shore path and a designated bathing lawn on the south-west shore away from the island crowds.	Lake	Gorenjska, Slovenia	29.5 m	3 min	4.5°	23°
Bains des Pâquis, Geneva A wooden bathing establishment built on stilts over Lake Geneva in 1872 — lake water, a 100 m protected swimming area, and a fondue restaurant on the same pier.	Lido & pool	Geneva, Switzerland	6 m	4 min	7°	22°
Lac d'Annecy, Plage de la Brune The cleanest large lake in France, fed by Alpine snowmelt and filtered through a limestone aquifer — clear enough that the 30 m depth line is visible from a paddleboard.	Lake	Haute-Savoie, France	65 m	2 min	5°	21°
Berounka at Branov A slow, sandy-bottomed river bend beside a wooden ferry landing, with the warmest summer water of any Czech river swim on this list.	River	Central Bohemia, Czechia	2.2 m	2 min	2.5°	20°
Hampstead Heath Mixed Pond A 1.2 hectare unheated bathing pond on Hampstead Heath, fed by spring water and opened to both sexes in 2004 — London's only mixed outdoor bathing pond.	Lido & pool	London, England	3 m	10 min	5°	20°
Tooting Bec Lido The largest freshwater outdoor pool in England by surface area — 91 m long, 30 m wide, unheated since 1906, and home to the South London Swimming Club's winter race series.	Lido & pool	London, England	2 m	5 min	4.5°	19.5°
Walchensee, Urfeld shore The deepest large lake in the German Alps — 190 m — and the reason it stays turquoise: a huge depth of water absorbs red light and reflects blue back.	Lake	Bavaria, Germany	190 m	4 min	4°	19.5°
Upper Rhine at Rheinau A broad, fast, island-fringed stretch of the Rhine where the standard local swim is a 1.2 km downriver drift between two bridges.	River	Schaffhausen, Switzerland	6.5 m	5 min	4°	19°
River Wye at Symonds Yat A wide, slow, meandering reach below a limestone gorge, with a shingle beach and a ferry that has run on a rope since the 18th century.	River	Herefordshire / Gloucestershire border, England	3.5 m	3 min	5.5°	18°
River Dart at Newbridge A wide, shingle-bedded bend of the Dart where a medieval bridge pins the flow into a long, slow pool — the most forgiving river entry in the Dartmoor ring.	River	Devon, England	2.6 m	4 min	6.5°	17.5°
Bude Sea Pool A 91 m tidal pool cut into the rocks below Summerleaze Down in 1930 — filled by every high tide, sheltered from the Atlantic swell, and free to use.	Sea	Cornwall, England	3.4 m	5 min	9°	17°
Buttermere The Lake District's most swimmable lake: no motorboats, no development, a 7 km shoreline path, and a shingle spit that gives deep water ten metres from the bank.	Lake	Lake District, England	23 m	6 min	3°	17°
River Wharfe at Grassington A limestone-bedded Dales river with a long shallow run and a deep pool under the old packhorse bridge — the Wharfe was England's first river to get inland bathing-water status.	River	Yorkshire Dales, England	3 m	8 min	5°	16.5°
The Forty Foot, Sandycove A granite bathing place at the tip of a Dublin headland, swum from year-round since 1740, immortalised in Ulysses, and still the benchmark for cold-water swimming in Ireland.	Sea	County Dublin, Ireland	6 m	8 min	9°	16°
Loch Morlich A shallow, sandy-bottomed loch at 300 m altitude with a pine-fringed beach — the only place in Britain where you can swim in a Cairngorm loch with a beach that would pass for a coastal one.	Lake	Cairngorms, Scotland	12 m	3 min	2.5°	15.5°
Verzasca at Lavertezzo Green-white granite pools under the Ponte dei Salti, the clearest river water in Switzerland and one of its most dangerous, because clarity makes depth impossible to judge.	River	Ticino, Switzerland	8 m	3 min	5°	15.5°
Llyn Idwal A glacial cwm lake at 380 m in a rock bowl under Y Garn — a 25-minute walk in, no roads, no boats, and water so clear you can see the boulders at 6 m.	Lake	Snowdonia (Eryri), Wales	12 m	25 min	2°	14.5°
Soča at Bovec The most photographed river in the Alps: an unnervingly clear turquoise channel over white limestone, cold enough in August to make a wetsuit worth carrying.	River	Julian Alps, Slovenia	3.2 m	12 min	3.5°	14.5°

Before you get in

Cold water is the one hazard that applies everywhere in this library. Below 15 °C the gasp reflex can make you inhale water involuntarily within seconds of immersion; below 10 °C, unacclimatised swimmers lose the ability to swim effectively within minutes. Enter gradually, keep your first session under five minutes, never swim alone, and get out while you still feel fine — the afterdrop continues after you leave the water.