Cold water kills strong swimmers in minutes, and it kills them long before hypothermia is even possible. Most people who die in UK waters were never trying to swim far — they fell in, gasped, and inhaled. This gives you the timeline for a given water temperature, the protocol that actually works, and what to do if you are the one on the bank.
Cold water shock — the first minute. Sudden immersion in water below about 15°C triggers an uncontrollable gasp reflex and a spike in heart rate and blood pressure. If your head is under when you gasp, you inhale water. This alone accounts for a large share of drownings, and it happens before you have swum a single stroke. It passes in 60 to 90 seconds if you can keep your face clear and control your breathing.
Cold incapacitation — 3 to 20 minutes. Cold pulls blood into your core to protect your organs, which cools the nerves and muscles in your arms and legs. You lose the ability to grip, swim or climb out. This is the stage most people never make it past, and it is why "I am a strong swimmer" is not protection — it is a muscular failure, not a fitness test.
Hypothermia — 30 minutes and beyond. Only if you survive the first two does core temperature actually start falling. Even in very cold water this takes half an hour or more, which is why people who drown in cold water are usually not hypothermic at all.
Afterdrop. Core temperature keeps falling for some time after rescue, as cold blood from the limbs returns to the centre. Handle a casualty gently, keep them horizontal, dry and covered, and do not rub their limbs or put them in a hot bath. Someone who was cold in the water can deteriorate on dry land, and they need hospital assessment even if they say they are fine.