Those post-sinking scenes were shot in a 350,000-gallon tank. To get the "frozen" look, crew members applied a special powder to the actors that crystallized upon contact with water, along with wax applied to clothes to look wet.
The making of Titanic was just as dramatic as the film itself. _cb01_gold_Titanic_1997
James Cameron didn’t just want to make a movie; he wanted to explore the wreck himself. The project was partly motivated by his desire to fund deep-sea dives to the actual Titanic site. Those post-sinking scenes were shot in a 350,000-gallon tank
While the Titanic is a historical catastrophe, the story of Rose DeWitt Bukater (Kate Winslet) and Jack Dawson (Leonardo DiCaprio) is purely fictional, designed by Cameron to personalize the disaster. _cb01_gold_Titanic_1997