One of the most common questions we get at Really Wild Swimming is some version of: “How do I actually know if the water is okay to swim in?” It’s a brilliant question, and the honest answer is that it takes a few different checks — not just one. Here’s how to build a quick but solid pre-swim routine.
Start With Swimfo — the Environment Agency’s Official Checker
If you’re swimming at or near a designated bathing water in England, your first stop should be Swimfo (environment.data.gov.uk/bwq/profiles). It’s the Environment Agency’s free service, and it’s genuinely useful.
Swimfo rates designated sites as Excellent, Good, Sufficient, or Poor, based on bacteria levels measured over a rolling four-year period. During the bathing season (mid-May to end of September), the EA takes weekly water samples and issues daily pollution risk forecasts at many sites — so you can see not just the annual rating but today’s risk level.
For Wales, Scotland, and Northern Ireland, check with your respective agencies: Natural Resources Wales, SEPA, and DAERA all publish equivalent monitoring data.
A few things worth knowing about Swimfo:
- Annual ratings are based on up to four years of data — so a site rated “Excellent” overall can still have a bad day after heavy rain.
- Only designated bathing waters are routinely monitored. If your favourite secret spot isn’t on the list, it won’t be there.
- Always check on the day, not just once a season. Conditions change.
Download the Surfers Against Sewage App
For real-time sewage alerts, the Safer Seas & Rivers Service app from Surfers Against Sewage is essential. It’s free, and it covers pollution alerts for over 450 locations across the UK — coasts and inland waters alike. The app tracks live sewage overflow data and flags when discharges are happening near your swim spot, giving you the clearest available picture of what’s actually in the water right now.
It also includes tide times and lifeguard information, which makes it a useful all-round planning tool, not just a water quality checker.
Check the Weather — Especially Rainfall
This is the step most swimmers skip, and it matters enormously. Heavy rain washes bacteria, agricultural runoff, and sewage overflow into rivers, lakes, and the sea. The general rule of thumb: avoid swimming for at least 48 hours after significant rainfall, particularly at river and estuary spots. Storm overflows discharge sewage into waterways after heavy rain — which is exactly why many sites flag whether their water quality is affected by rainfall.
The Environment Agency actually uses Met Office data to generate pollution risk forecasts at designated sites, so checking Swimfo after a rainy spell will often show a temporary downgrade even at normally excellent spots.
Use Your Own Eyes and Nose
Official data can only tell you so much. The Outdoor Swimming Society puts it simply: if the water looks unappealing — scummy, cloudy, or discoloured — or if it smells wrong, trust your instincts and stay out.
Know what you’re looking for:
- Blue-green algae — look for greenish, blue-green, or brownish scum, especially near shorelines and in still or slow-moving water in warm weather. It tends to be more common in lakes and reservoirs in summer. The key thing to know: you can’t tell whether a bloom is toxic just by looking at it, so the safest approach is to assume it is and avoid the water entirely. Contact can cause skin rashes, eye irritation, vomiting, and diarrhoea — and it poses a serious risk to dogs.
- Dead fish or wildlife in or around the water — a clear warning sign of a pollution event.
- Strange foam or oily sheen on the surface.
- Unusually murky or discoloured water after rainfall — runoff can carry a lot of nasties.
Know Your River Ratings
For non-designated river spots, the Environment Agency’s water quality map gives historical chemistry and biology ratings (graded A–F) for monitoring points across England and Wales. It’s not real-time and won’t test for bacteria like E. coli, but it gives useful background context. As a rough guide: ratings of A or B mean good or very good water quality. Ratings of D, E, or F? Don’t swim there. A C-rated site warrants caution — cover any cuts and try to keep your head above water.
You can also check whether your swim spot is upstream or downstream of sewage treatment works, intensive farms, or industrial sites using an OS map or the Rivers Trust’s interactive sewage outflow map. Swimming upstream of these is always the better call.
After Your Swim
Even with all the right checks done, open water will always carry some micro-organisms. A few basic habits help keep you well: shower or wash off as soon as you can after swimming, keep your mouth closed if you go under, cover any cuts before you get in, and wash your hands before eating or drinking. If you develop symptoms like stomach upsets, skin rashes, or ear or throat infections in the days after a swim, it’s worth mentioning to your GP that you’ve been wild swimming.
The Quick Pre-Swim Checklist
- Check Swimfo for your site’s rating and any current pollution warnings
- Check the Safer Seas & Rivers Service app for real-time sewage alerts
- Did it rain heavily in the last 48 hours? If yes, consider waiting
- Scan the water visually — any scum, discolouration, or odd smell?
- Look out for blue-green algae warning signs posted at the site
- Are you upstream or downstream of any obvious pollution sources?
None of this needs to take more than five minutes, and it becomes second nature quickly. The goal isn’t to put you off swimming — it’s to help you swim with confidence, knowing you’ve done the sensible groundwork. Happy swimming.
