I arrived at a beach on a hot day to find a sign advising against swimming, nobody in the water, and no explanation of what the problem was. Nobody I asked seemed to know either.
Learning what these notices actually mean has made me both more careful about some and considerably less worried about others.
The categories are different and get conflated
A beach closure or advisory can be about water quality, sea conditions, or a specific hazard, and they are entirely different things with different levels of seriousness.
Water quality notices are about bacterial contamination, usually from sewage or agricultural runoff.
Flags are about physical conditions — waves, currents, whether lifeguards are present.
And hazard notices cover specific temporary problems: jellyfish, algal blooms, pollution incidents, occasionally something washed up.
Treating all of these as the same thing leads people to ignore all of them, which is the wrong response to at least two of the three.
Water quality, and the reason it is worst after rain
The pattern that is worth understanding because it lets you predict it.
In many places, heavy rainfall overwhelms drainage and sewage systems, and the overflow discharges into rivers and the sea. Agricultural land also sheds material into watercourses during heavy rain.
Which means water quality is frequently at its worst in the day or two after significant rainfall, and at its best after a dry settled spell.
Routine monitoring happens through the bathing season in many countries, with results published, and beaches are classified on the basis of samples taken over time.
The gap in that system is that sampling is periodic and pollution incidents are episodic. A beach with an excellent classification can be badly affected on a particular day and the sample may not catch it.
The practical rule most people who swim regularly follow is to avoid swimming for a day or two after heavy rain, particularly near a river mouth or a town, regardless of what any sign says.
The flag systems
Broadly similar internationally and not identical, and the differences matter.
Red generally means do not enter the water. In some systems it means dangerous conditions and in others it means the beach is closed entirely.
Yellow generally means caution, with conditions requiring care.
Green means conditions are considered suitable, which is not the same as safe.
A chequered flag in many systems marks an area for craft rather than swimmers, which people misread as a swimming zone.
And no flags at all does not mean fine. It means unpatrolled, which is a different statement entirely and is the situation on most of the world's coastline.
Most beaches display an explanation of their own system near the entrance, and reading it once on arrival takes a minute.
Jellyfish notices
Where the seriousness varies enormously by species and region.
In most temperate waters, the species encountered produce stings that are painful and not dangerous, and a notice is genuine information rather than a serious warning.
In some regions there are species that are genuinely dangerous, and local warnings about those should be treated as absolute. This is one of the situations where the standard travel advice to follow local guidance is not a formality.
Seasonal patterns are often predictable and known locally, driven by water temperature and currents, and asking is more useful than guessing.
Vinegar is commonly recommended for some species and is actively harmful for others, which is why the correct first aid depends entirely on where you are. Knowing the local advice before you need it is the useful preparation.
Algal blooms
Less commonly discussed and worth knowing about.
Some blooms are harmless discolouration. Some produce toxins that affect people through skin contact, through inhalation of spray, or through shellfish that have concentrated them.
Shellfish is the significant one. Harvesting closures during blooms are a serious matter and gathering your own shellfish during one can produce genuine illness.
Commercial supply is monitored for this. Gathering personally in an area you do not know well is where the risk sits.
What I do
Check the published water quality classification for the beach before a trip, which takes two minutes for most European and North American coasts.
Not swim for a day or two after heavy rain.
Read the flag explanation on arrival and take red flags seriously regardless of how the sea looks, because the reason is frequently a current that is invisible.
Ask locally about jellyfish and anything else seasonal.
And accept not swimming on some days, which happens a few times a year and has never once turned out to be a loss.
Where to look before you travel
Most countries with a bathing water regime publish results online, per beach, updated through the season.
These give a classification based on sampling over several years, plus recent individual results, and they are free and searchable.
Some also publish real-time pollution alerts, which are considerably more useful than the classification because they tell you about today rather than about the last four summers.
Checking the classification once when choosing a destination, and the alerts on the morning, covers both the structural and the immediate question in about three minutes.