what should you do if you fall overboard into cold water?

If you fall overboard into cold water, your priorities are to control your breathing, get as much of your body out of the water as possible, and conserve heat until you can be rescued. Staying calm, keeping your lifejacket on, and using heat-saving body positions can dramatically increase your survival time.

Immediate first seconds: cold shock

• Try to stay calm and fight the instinct to thrash or swim hard; focus only on keeping your head above water and breathing slowly.

• Cold shock usually lasts 1–3 minutes, during which you may gasp uncontrollably and hyperventilate; keep your airway clear and avoid inhaling water.

• If you are wearing a lifejacket or PFD, trust it to keep you afloat and use that time to regain control of your breathing.

Next minutes: get out and stay afloat

• Once breathing is under control, try to get out of the water quickly: climb back onto the boat (even if capsized or swamped) or any floating object to raise most of your body out of the water.

• Only swim for shore if it is very close and you are certain you can reach it quickly; unnecessary swimming greatly speeds up heat loss and exhaustion.

• Keep your lifejacket on and do not remove clothing unless it hinders your movement; clothes can trap a layer of water and help insulate you.

Heat conservation positions

• If alone, use the H.E.L.P. (Heat Escape Lessening Position) : cross your arms tightly over your chest, pull your knees up to your chest, and keep your head and face out of the water to reduce heat loss from your core.

• If you are with others and cannot get out of the water, huddle together chest-to-chest, arms around each other’s shoulders and sides, keeping everyone’s heads above water to share warmth and improve visibility.

• Minimize movement; the more you move or try to swim without a clear goal, the faster you lose heat and strength.

If you stay in the water longer

• Watch for signs of hypothermia in yourself (or others once rescued): intense shivering, clumsiness, confusion, and eventually slurred speech or drowsiness.

• If rescued, you or the rescuer should focus on gentle rewarming: remove wet clothing, wrap in dry clothes or blankets, warm the torso first, avoid direct intense heat (like very hot baths), and never give alcohol.

• Seek medical help as soon as possible after any prolonged cold-water immersion, even if you feel “okay,” because dangerous complications can develop later.

TL;DR:
Control your breathing, keep your lifejacket and clothes on, get as much of your body out of the water as you safely can, avoid unnecessary swimming, use H.E.L.P. or a huddle to conserve heat, and get medical help as soon as you are rescued.

Information gathered from public forums or data available on the internet and portrayed here.