A sign for Windermere, Millerground Landing

Print this page on A4, portrait, and put it up where swimmers will see it, with the landowner's or the venue's agreement.

It gives no level: printed, a level would be out of date within hours. The QR code and the address open the spot's forecast, which is updated several times a day.
It needs no colour printer. Print at actual size; a printer's "fit to page" makes it slightly smaller, which still scans.
For a screen at the venue, the live sign shows the forecast and updates itself. The organisers' page lists the overflows upstream.

SwimSignal

Windermere, Millerground Landing

A five-day pollution risk forecast for this spot, from the water companies' live sewage-overflow data and rainfall forecasts.

Scan for the forecast, or go toswimsignal.co.uk/spot/bw-ukd1203-45650

Not a safety check: look at the signs at the water before you swim.

SwimSignal is independent: not run by or connected with the Environment Agency or any water company. Data credits: swimsignal.co.uk/terms.html#data