Thanks! By vasodilation the alcohol must have enabled him to keep swimming and producing fresh heat. Without alcohol the body gives up on the extremities first.
Wiki:
The actual cause of death in cold water is usually the bodily reactions to heat loss and to freezing water, rather than hypothermia (loss of core temperature) itself. For example, plunged into freezing seas, around 20% of victims die within two minutes from cold shock (uncontrolled rapid breathing, and gasping, causing water inhalation, massive increase in blood pressure and cardiac strain leading to cardiac arrest, and panic); another 50% die within 15–30 minutes from cold incapacitation: inability to use or control limbs and hands for swimming or gripping, as the body "protectively" shuts down the peripheral muscles of the limbs to protect its core.[39]
So without moving I assume alcohol would just accelerate hypothermia.
The only thing that now worries me are side effects like confusion before losing consciousness.