The passengers of the Titanic that did not make it into a lifeboat had twelve minutes to swim to a lifeboat. After twelve minutes, hypothermia would kill them. The water of the North Atlantic in April is so cold, that a person will die, if they spend more than twelve minutes in that water. The Titanic took a long time to sink. The lifeboats were well aware of the danger of being sucked down, when the big ocean liner sank, so they moved away from the ship. This also got the lifeboats away from the people swimming in the water. They were afraid that the lifeboats would become over loaded and sink, if they pulled to many people from the water.
Source: "Lost Liners" by Robert Ballard.
"A Night to Remember" by Walter Lord