Looks Over Function
Did you know that Kate Winslet, who played Rose, almost drowned on set? During one of the tidal wave scenes, the wave’s force pushed her underwater, and her jacket got caught. It took several agonizing moments before she was eventually freed. This is probably one of the reasons why she isn’t wearing a jacket in the movie.
But everyone else was, which makes it completely unrealistic for her not to wear one. Also, James said that the jacket was too bulky, which would’ve only exposed her even more to the freezing water and air. If she didn’t drown while trapped in the lower deck, she certainly would’ve died of exposure.