The best method of treating hypothermia and / or cold shock is to prevent it from happening in the first place.
Dress appropriately for the water temperature regardless of the air temperature. A general rule of thumb is to wear wet or dry suits when water temperatures is less than 50°F.
The head is a primary heat loss area
Wool, nylon or synthetics will not prevent cold shock
Carry dry extra clothing
Carry a sleeping bag or space blanket
Always remember your ABC's when assessing some one for cold related injuries. Their AIRWAY must be open, they must be BREATHING and their must be blood CIRCULATING, otherwise your efforts are for naught!
Learn CPR if you don't know already. Your training may save a life!
Gently remove cold, wet clothing. Place into dry clothing, sleeping bag, wrap with space blankets. Prevent further conductive or convective heat loss.
External heat - chemical or other heat packs to armpits, groin, around neck, scalp (near major blood vessels), body to body contact (share your heat but must be skin to skin contact).
Warm Liquids - Small sips of warm not hot liquids.
Avoid alcohol - A vasodilator, alcohol causes blood vessels to dilate and releases cold blood from the extremities into the core. May precipitate heart arrhythmias and sudden death.
Move gently and only horizontally - Avoid after drops of blood pressure and release of cold blood to the core. Sudden, vertical position changes can cause after drop and sudden death.
Seek medical attention as soon as possible.