Open-water swimming

Is open-water swimming safe?

For a fit adult who swims in a group, in water they know, it is a modest risk: we estimate about 0.5 to 2 micromortsMicromortA one-in-a-million chance of death. It lets you compare the risk of very different activities on one scale.More in the glossary per swim, similar to a day on the ski slopes or a 100-mile drive in the US. The risk rises sharply in three settings. Cold water in the first minute after entry, crowded race starts, and swims by older men with undiagnosed heart disease account for a large share of deaths.

There is no official per-swim death rate, so our figure is built from two numbers. The UK's Water Incident Database (WAID) recorded 189 accidental deaths of people swimming outdoors in 2019–2023, about 38 a year. Sport England's Active Lives survey for 2022–23 found about 2.45 million adults in England who mostly swim in open water and 1.92 million more who use both open water and outdoor pools. If those 4.4 million or so swimmers make 5 to 10 swims a year each, the risk is roughly 0.5 to 2 micromorts per swim. The deaths include holidaymakers who were not regular swimmers, so a practised swimmer is likely at the lower end.

Cold water shock, the first minute

Most people imagine drowning as slow exhaustion far from shore. In cold water the danger comes early. Sudden immersion triggers the cold shock response: a gasp, then fast, uncontrolled breathing and a jump in heart rate and blood pressure. A gasp with your face in the water can mean inhaling water. The surge can also trigger heart rhythm problems in people with an underlying condition.

The University of Portsmouth's Extreme Environments Laboratory has studied this for decades. A 2024 meta-analysis led by its researchers found that the response weakens with repeated exposure, typically after about four immersions, with the heart rate jump falling by about 14 beats a minute. That is the case for acclimatising gradually through spring rather than diving into a cold lake on the first warm day of the year. UK drownings peak in warm months, in 2025 August had the most, when air temperatures tempt people into water that is still cold.

Race swims and the heart

Organised events carry their own pattern. Harris and colleagues compiled 135 deaths and cardiac arrests in US triathlons from 1985 to 2016. Ninety happened in the swim. In USA Triathlon races from 2006 to 2016 the rate was 1.74 per 100,000 participants, 2.40 for men and 0.74 for women. For men aged 60 and over it was 18.6 per 100,000.

Taking two-thirds of the events as swim-related gives roughly 12 per million starts, or about 12 micromortsMicromortA one-in-a-million chance of death. It lets you compare the risk of very different activities on one scale.More in the glossary for a race swim. That is several times the risk of a quiet swim with friends. Of the decedents who had an autopsy, 44% had clinically relevant heart disease, mostly coronary disease or cardiomyopathy. The mass start, with contact, cold and adrenaline, loads the heart at the moment a swimmer is hardest to see. For a comparison with another endurance event, see marathon running, where heart events are also the main killer.

Where swimmers drown

In the UK, more than half of swimming deaths in 2019–2023 happened at the coast, where rip currents, offshore winds and tides catch people out. Across all accidental drownings, inland water (rivers, lakes, quarries and reservoirs) is the most common location: 57% in 2025. Rivers carry hidden currents and cold water from upstream, and quarries are often deep and very cold below the surface layer.

Men make up about 85% of accidental drowning deaths in the UK, and in 2025 men aged 60 to 69 had the highest rate. Alcohol or drugs were noted in 41 of the 202 accidental deaths that year.

How to swim outdoors more safely

1. Get in slowly. Walk in, splash your face and wait for your breathing to settle before you put your face in or swim away.
2. If you gasp or panic, float. Lie back and let the cold shock pass before you do anything else.
3. Acclimatise over weeks, starting with short swims in spring, and keep early-season swims short.
4. Never swim alone. Use a supervised venue or swim with someone watching from the shore or a kayak.
5. Use a bright tow float and swim cap so boats and rescuers can see you.
6. At the beach, swim between the red and yellow flags where there is a lifeguard. If a rip current pulls you out, do not swim against it. Float, or swim parallel to the shore until you are out of it.
7. Skip alcohol before and during a swim.
8. Before your first triathlon or long swim, especially over about 50, talk to your doctor about heart checks, and practise in a wetsuit in open water before race day. Start at the side or back of the pack.

How we scored it

Fatality risk (72) reflects about 1 micromortMicromortA one-in-a-million chance of death. It lets you compare the risk of very different activities on one scale.More in the glossary for a typical swim and much more for race starts. Injury risk (82) is good, since non-fatal harm is uncommon. Controllability (70) is fair: acclimatisation and company help, but cold shock and hidden heart disease do not respond to skill. The rescue environment (50) is poor, as most swims have no lifeguard. Scuba diving scores a little higher, with structured training and a buddy system that most open-water swimmers lack.