Hypothermia is likely the least of your problems after an involuntary dip into icy water. Drowning and cardiac arrest are the killers here.
Read these, and maybe watch the videos. It's all good stuff, and difficult to summarise.
http://beyondcoldwaterbootcamp.com/4-phases-of-cold-water-immersion
http://beyondcoldwaterbootcamp.com/en/rescue-a-treatment
Basically, adults need over 30 mins in icy water before you get even mildly hypothermic. From beyondcoldwaterbootcamp:
While it varies with water temperature and body mass, it can take 30 minutes or more for most adults to become even mildly hypothermic in ice water.
http://sportsscientists.com/2008/01/exercise-in-the-cold-part-ii/
Perhaps the first, and maybe the most surprising fact about cold water physiology, is that your body has too much heat to become hypothermic within about 30 minutes, no matter how cold the water is!
... and from @imsodin's link:
Even in ice water hypothermia does not set in until after 20 to 30 minutes, depending on the amount and type of clothing.
If you're out of the water and wet, you will lose heat by evaporation (as well conduction and convection), so dry clothing and dry body are important.
