Most people who get into difficulty in the sea on holiday are competent swimmers. The problem is rarely ability, it is unfamiliarity with a specific stretch of water, and the things that make one beach dangerous are frequently invisible.
This is a general description rather than safety instruction. Local advice always takes precedence over anything written elsewhere.
Ask somebody who is there every day
The most useful thing available and the most underused.
Lifeguards, where present, know that beach in detail — where the currents run, where the bottom shelves suddenly, what the tide does. They are generally happy to be asked and almost nobody asks.
Where there is no lifeguard, the people who run beach businesses know, and so do people fishing.
The question that gets the useful answer is not whether it is safe. It is where they would swim and where they would not.
Flags and what they mean locally
Flag systems are broadly similar and are not identical between countries, and the differences matter.
Red generally means do not enter, and in some places means dangerous conditions while in others means the beach is closed entirely.
Yellow generally means caution. What caution refers to varies.
The one that catches people is the absence of flags, which does not mean safe. It means unpatrolled, which is a different statement.
Most beaches display an explanation of their own system somewhere near the entrance. It takes a minute to read.
What a rip current looks like
The single most common cause of difficulty and it is identifiable from the shore if you know what to look for.
A channel of water moving away from the beach. It often appears as a gap in the line of breaking waves, a stretch of choppier or discoloured water, or a section where foam and debris are moving seaward.
Counterintuitively, the calmer-looking gap between breaking waves is frequently the dangerous part, and people choose it because it looks easier.
Standing and watching the water for five minutes before going in is enough to see the pattern, and it is the single most useful thing anybody can do on an unfamiliar beach.
The standard advice if caught in one is not to swim against it, since it is faster than you are, but to swim parallel to the shore until out of the channel, then in. Local guidance may differ and should be followed.
The things specific to unfamiliar places
Tidal range varies enormously by location. In some places the water moves hundreds of metres and the timing matters for anything on a sandbar or around a headland.
Water temperature is a real hazard and is underestimated in warm climates, where air temperature is no guide. Cold water shock affects strong swimmers.
Marine life varies and local knowledge is the only useful source. Some places have seasonal hazards that are entirely routine to residents and unknown to visitors.
And the bottom. Sudden drop-offs, rocks under the surface, and strong undertow at steeply shelving beaches are all invisible from above.
The pattern in the incidents
Reading accounts of what actually happens, the same factors recur.
Swimming alone, or at a time when nobody else is present.
Alcohol, which is involved far more often than people expect and which also affects the decision to go in at all.
Going in after somebody else who is in difficulty, which is how a substantial proportion of drownings involve people who were on the shore. Getting help or throwing something that floats is more effective and considerably safer.
And swimming outside patrolled areas or hours, which is the single strongest predictor in every set of statistics I have looked at.
Children
Worth its own paragraph. The consistent finding is that supervision needs to be continuous and close, and that difficulties happen quickly and quietly rather than dramatically.
The practical arrangement that works is one adult designated as watching, with the role explicitly handed over rather than assumed. Groups where everybody assumes somebody else is watching are the situation that recurs in accounts of incidents.
What I do
Watch the water for a few minutes before going in, every time, even somewhere I have been before, because conditions change.
Swim where other people are swimming, and between the flags where they exist.
Not alone, and not in the evening when the beach has emptied, which is when it is most tempting.
Ask somebody local on the first day.
And accept not swimming on days when it does not look right, which happens perhaps twice a year and has never once felt like a loss afterwards.
Pools, rivers and lakes
Briefly, because the attention goes to the sea and the other water is where a good deal goes wrong.
Rivers have currents that are not visible from the bank, and depth that changes without warning. Water that looks slow is frequently not.
Lakes and quarries are colder than they look, sometimes dramatically so a short distance below the surface, and cold water shock is the specific hazard.
Hotel pools are a common site of incidents involving children, generally where supervision was assumed rather than assigned.
The common factor across all of it is unfamiliarity, which is the same reason unfamiliar sea is riskier than familiar sea for the same swimmer.