How this works, and what it can't tell you

Every half hour, this site reads the official warnings and the live sewage overflow feeds for all 941 designated bathing waters in Britain and Ireland, and works out a single answer for each one.

How a verdict is decided

Do not swim today — somebody official has said so about this water: bathing is prohibited, a pollution incident is open, or a monitored storm overflow is discharging within 2km right now.
Advised against today — the regulator's daily pollution risk forecast advises against bathing here. It is a model's view of today, usually after heavy rain, rather than a measurement or a notice — which is why it sits below the level above it.
Take care — the regulator forecasts increased pollution risk, or an overflow discharged nearby in the last 48 hours, or heavy rain has fallen at a beach known to be affected by it.
No warnings today — the checks that could be made were made, and none of them found anything.
Can't say today — something could not be checked: a feed was down, a nearby overflow monitor was not reporting, or the bathing season has ended and the daily forecasts have stopped.

Each beach lists what was actually checked and what wasn't, so a green tick is never a bare assertion. Four rules the code holds to:

Never show green because something failed. A dead monitor, a feed that is down or a season that has ended all produce “can't say”.
An official warning outranks anything calculated here.
Last season's classification is not evidence about today's water. It can add a warning; it never earns a green tick on its own.
Say when, and say whether a number was measured or modelled.

What it can't tell you

Ireland has no sewage spill monitoring at all, north or south. There is no monitoring programme to read. For Irish beaches this shows official notices and sample results only — an empty spill list there means nobody is measuring.
Scotland forecasts 30 of its 90 beaches. The rest have last year's rating and nothing about today.
The season ends. Daily forecasts and weekly sampling stop in mid-September in Scotland and Ireland, and at the end of September in England and Wales. Out of season the site says so rather than showing green.
Only monitored overflows appear. Unmonitored pipes, farm run-off, misconnected drains and boats do not. A monitor that has stopped reporting counts as unknown, never as clean.
Distance is not a model. A spill within 2km counts against a beach; tides and currents decide what actually reaches the water. Southern Water's own tidal assessment is used where they publish it.
Rainfall is modelled, from a weather model on roughly an 11km grid — not a rain gauge at the beach.

Where the data comes from

Environment Agency — daily pollution risk forecasts, open pollution incidents, suspensions and classifications for England
Natural Resources Wales — the same for Wales
SEPA — Scottish classifications and daily predictions
DAERA — Northern Irish sampling
EPA Ireland and local authorities — classifications and statutory bathing restrictions
Thames, Southern, South West, Wessex, United Utilities, Yorkshire, Northumbrian, Anglian, Severn Trent, Dŵr Cymru Welsh Water and Scottish Water — live storm overflow status for 18,600 monitored outfalls
Open-Meteo — rainfall

The code that collects it is public: Leiglow/swim-collector.

This is not an official source

It reads published data and presents it plainly. It is not a substitute for the regulator, the water company or a sign at the beach. If a sign says otherwise, believe the sign. Nothing here is advice about whether you personally should swim.