March · 7.1 °C

21 published records ordered by mean surface temperature in March, spanning 3–13 °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 March
Spot	Type	Region	Max depth	Walk	January	August
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°
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°
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°
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°
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°
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°
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°
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°
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°
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°
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°
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°
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°
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°
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°
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°
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°
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°
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°
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°
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°

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.