A warm sea and a calm-looking surface are poor predictors of safe swimming. The conditions that matter most are largely invisible from the shore.
Rip currents form where water must escape
Waves push water toward the shore continuously, and that water has to return seaward somewhere, usually through a channel in the sandbar.
The returning flow is concentrated and fast, and it appears at the surface as a calmer, darker gap between breaking waves.
That deceptive calm is why swimmers enter there, and it is the single most common mechanism behind beach rescues on surf coasts.
Wave shape matters more than wave height
Waves breaking gradually along a shallow slope dissipate energy over distance, which is why gently shelving beaches feel manageable even with sizeable surf.
Waves breaking directly onto a steep beach release their energy at once, producing a heavy shorebreak that can injure people standing in shallow water.
A steep beach also produces a strong backwash that pulls the feet from under a swimmer at the moment the next wave arrives.
The seabed changes between visits
Sandbars and channels shift with storms, so the safe entry point on one visit may sit over a rip channel the following month.
This is why local knowledge outperforms memory, and why lifeguards reposition their flags rather than leaving them in a fixed place.
Flags mark the area assessed as safest that day, and the assessment is remade each morning against the conditions actually present.
Cold water is a hazard independent of air temperature
Water conducts heat away from the body far faster than air does, and a sea that feels refreshing on entry can impair coordination within a surprisingly short time.
Sudden immersion in cold water also produces an involuntary gasp and rapid breathing, which is dangerous specifically because it occurs at the moment of entry.
Anyone swimming in genuinely cold water should be seeking guidance from people who do it locally, since acclimatisation and technique are not things to improvise.
How to read a beach before entering
Watching the water for several minutes reveals the pattern of sets, the gaps between breaking waves and where the water is moving seaward.
Posted flags and signage encode local hazards, and the meaning of flag colours differs between countries, so it is worth confirming rather than assuming.
Where lifeguards are present, asking them directly is faster and more reliable than any of this, and it is a routine part of their work.