The strongest result was not that swimming outdoors felt good. People rated both outdoor pools and natural waters very positively. The surprise was that the setting still appeared to matter after the researchers tried to account for who was swimming, when they went and what the visit was like.
Across 1,200 recently recalled swimming visits, positive wellbeing averaged 2.25 for swims in oceans, lakes and rivers, compared with 1.94 for open-air pools on a scale running from minus three to plus three. The authors described that difference as about 9 per cent.
The natural-water advantage was associated mainly with two experiences: feeling free to be oneself and feeling a sense of achievement. Social closeness was higher during open-water visits too, but it did not explain the wellbeing difference in the researchers’ mediation model.
That is more precise than the familiar claim that wild swimming works because of cold, community or nature alone. It is also observational evidence based on memory, not a trial showing that moving the same person from a lido to a lake will improve their mental health by 9 per cent.
One survey produced 1,200 swimming visits
The study in the Journal of Environmental Psychology drew from the BlueHealth International Survey, a much larger project on contact with green and blue spaces. The full survey included 21,352 adults across 19 countries.
Data were gathered in 14 European countries and in Hong Kong, Canada, Australia and California during four seasonal waves in 2017 and 2018. An Austrian sample was added in October 2020. The paper appeared in 2025, so “a 2025 study” describes the publication date rather than the age of every swimming experience.
Participants were asked about recreational visits to 17 kinds of blue space during the previous four weeks. Anyone who had made such a visit described the most recent one and identified the main activity. The analysis selected the 1,200 people who said that activity was swimming.
Of those visits, 845 took place in open water. That category included coastal settings, lakes, reservoirs, rivers and canals. The remaining 355 occurred in an outdoor public pool, lido or thermal spa. Private pools were explicitly meant to be excluded.
The swimmers were almost evenly split by sex, with 611 women, and were unusually balanced across five age bands from 18 to 60 and older. Most visits, 61 per cent, occurred in late summer or early autumn.
What “9 per cent higher wellbeing” measured
The researchers did not administer a clinical mental-health assessment immediately before and after each swim. Participants looked back on one visit and rated their agreement with three statements: that they were satisfied with it, found it worthwhile and felt happy because of it.
Those answers were combined into a positive-wellbeing score. Each item used a seven-point response scale recoded from minus three to plus three. Both settings landed near the positive end, at 1.94 for open-air pools and 2.25 for open water.
The difference of 0.31 points is what the paper described as roughly 9 per cent higher wellbeing. It is a relative description of scores on the study’s scale, not a claim that wild swimming adds 9 per cent to a person’s mental health, life satisfaction or happiness outside that visit.
Open-water visitors reported significantly greater satisfaction, worthwhileness and happiness in the unadjusted comparison. Coastal and inland settings also produced very similar patterns when the researchers separated them in sensitivity analyses.
Anxiety was different. It was low in both groups and did not differ significantly by setting. Natural water was associated with more positive feelings, but the data did not show that open-water swimmers were less anxious during the visit than outdoor-pool swimmers.
Why compare a lake with a lido?
Many studies of “wild swimming” cannot separate the possible influence of nature from the effects of swimming, exercise, being outdoors or meeting other people. Comparing natural water with an open-air pool holds at least two of those ingredients relatively steady: both groups swam, and both did so outside.
The comparison is imperfect. A river and a heated lido differ in temperature, currents, scenery, rules, accessibility, perceived danger and the kinds of people who choose them. But an outdoor pool provides a more informative comparison than a sedentary control group or an indoor office.
The design also helps place ScienceBlog’s recent report on the immediate effect of looking at water in context. In that small experiment, watching a swimming pool for 100 seconds was associated with lower blood pressure than watching a tree or street sign. A human-made pool can still be calming and enjoyable.
The new study is therefore about added value, not a contest between healthy nature and joyless concrete. Outdoor-pool visits already received strong wellbeing ratings. Natural settings were associated with a modest additional difference.
Freedom carried most of the statistical explanation
The researchers organised their explanation around self-determination theory. This framework proposes that wellbeing is supported when people experience autonomy, competence and relatedness.
Each need was represented by one survey statement. Autonomy was measured by “I felt free to be who I am.” Competence was represented by a sense of achievement. Relatedness meant feeling closeness or intimacy with others.
Open-water swims scored higher on all three. After the researchers added them to a structural equation model, however, the indirect association with positive wellbeing ran mainly through autonomy. The standardised indirect effect was 0.43 for autonomy and a much smaller 0.05 for competence.
The equivalent estimate for relatedness was 0.01, with a confidence interval spanning zero. In plain language, open-water swimmers did report more social closeness, but that variable did not account for why their positive-wellbeing scores were higher.
That may seem surprising because community features heavily in accounts of outdoor swimming. The result does not say companionship is unimportant. It says that, within this model and these single-item measures, freedom and achievement better explained the difference between natural water and outdoor pools.
Mastery can contain a little unease
Natural water asks a swimmer to read wind, current, temperature, depth and an irregular shoreline. Navigating that uncertainty may create the sense of competence captured by the study’s achievement item.
Yet competence had a complicated relationship with anxiety. Greater autonomy was associated with less anxiety, while competence was associated with slightly more. The authors suggested that confident swimmers may select more demanding places, where skill and unease can rise together.
This is a useful reminder that wellbeing is not the same as comfort. A challenging experience can feel worthwhile and produce a sense of accomplishment without being uniformly relaxing.
It also makes a purely physiological story less convincing. The survey did not measure water temperature, cold-shock hormones, swimming intensity or time spent immersed. It cannot show that cold water caused the difference, and many of the open-water visits may not have been especially cold.
The researchers controlled for a great deal
The models adjusted for age, sex, income, minority status, limiting illness or disability and general wellbeing during the previous two weeks. They also considered whether adults or children accompanied the swimmer, weekday versus weekend, transport, visit duration, time of day, perceived water quality, season and country.
That matters because open-water swimmers were not interchangeable with pool swimmers. People reporting natural-water visits tended to have higher general wellbeing before the particular visit was considered. They were also wealthier on average, more likely to attend with other adults and less likely to be accompanied by children.
The adjusted association survived those controls. It also appeared in both coastal and inland open water. These checks make a simple demographic explanation less likely.
Statistical adjustment cannot remove every difference that was not measured well or measured at all. Swimming confidence, personality, motives, familiarity with a site and expectations about nature could influence both the choice of location and the rating afterwards.
Why the study cannot establish cause
The survey was cross-sectional, and each participant contributed one self-selected, recalled visit. No one was randomly assigned to a pool, lake or coast. There was no before-and-after measure tied to the swim and no follow-up showing how long any lift lasted.
Memory can favour an especially vivid sea swim over a familiar visit to the local pool. People who value freedom and outdoor challenge may choose natural water in the first place. Someone who had a frightening or unpleasant experience may be less inclined to describe swimming as the visit’s main activity or to repeat it.
The three proposed psychological mechanisms were measured with one statement each. That made an international survey practical, but autonomy, competence and social connection are richer than a single answer can capture. Mediation analysis can show that variables fit a proposed pathway; with cross-sectional data, it cannot prove the direction of that pathway.
The comparison groups were also unequal, with 845 open-water visits and 355 pool visits. Country samples were not large enough to make reliable nation-by-nation comparisons, and the availability of beaches, lakes and lidos varied sharply by place.
A stronger causal test would follow the same swimmers across different settings, collect ratings immediately before and after each swim, record temperature and conditions, and repeat the measurements across seasons. Random assignment would be difficult, but within-person studies could reduce many selection differences.
Water quality and safety are part of the result
Perceived water quality was associated with higher positive wellbeing in both settings. That supports the paper’s call for safe access to clean natural waters, but it should not be read as encouragement to enter any river or sea.
Open water adds currents, waves, cold shock, poor visibility, changing depth and fewer lifeguards. The CDC recommends checking local monitoring, advisories and closures, particularly after heavy rain, and staying out when water is discoloured, smells bad or shows signs of an algal bloom.
The RNLI advises swimming with a buddy, checking weather and tides, choosing a lifeguarded location where possible and entering cold water slowly. Conditions and appropriate precautions vary by country and site.
This is also where ScienceBlog’s earlier examination of sea air and immune signalling offers a useful boundary. A plausible biological response is not automatically a health benefit, and a natural setting is not automatically benign. Water quality, exposure and personal circumstances still matter.
A modest difference with an interesting explanation
The study does not show that an ocean is therapeutic, that cold plunges treat illness or that pools are second best. It shows that people remembered both kinds of outdoor swimming as highly positive, while natural-water visits carried a modest additional association with happiness, satisfaction and worthwhileness.
Its most valuable contribution may be the explanation. The statistical advantage of lakes, rivers and coasts was not mainly social. It was linked to feeling less constrained and more capable.
That finding gives future studies something concrete to test. If the effect is real, the benefit may come not simply from being surrounded by nature, but from what the setting allows a swimmer to feel about personal choice and ability.
For now, the honest conclusion is smaller than the cultural story surrounding wild swimming. Natural-water swimmers in a large international survey recalled slightly better visits than outdoor-pool swimmers, and freedom accounted for most of the difference. Whether the water created that feeling, or certain people and experiences found their way to that water, remains open.