The water temperature in Titanic's vicinity at the time of the collision late in the evening of the 14th was said to be in the upper 20s. Sea water freezes at a lower temperature (approximately 28.4 degrees F) than freshwater.
The extremely frigid waters expedited hypothermia among the mass of people adrift. Most died within minutes after plunging into the sea.
From AccuWeather.com (find the original story here); reprinted with permission.