I watched Titanic just last week as it was on TV and in my utter boredom sat down and watched.
Apparently, James Cameron left the ending open to the viewers imagination, stating that its up to the audience member to decide if she died or was just asleep and dreaming.
My interpretation was that she died
(a) when she's on the raft with Jack, he told her to go and live a long life and that she'd die an old lady, warm in her bed, the last scene was her in bed...
(b) she dropped the necklace back into the ocean, so it was like she was ready to let it go, instead of hold on to it
(c) when those doors open, she's reunited with Jack and all those who died on the ship, so it was like she was reuniting with them all in heaven.
I watched this film when I was quite young and never really understood the film that well. I remember some of my first questions were did she ever find out what happened to her mother? Before I realised, she never actually saw her mother again because all her remaining family believed that she went down with the Titanic when it sank, when in reality she changed her name and her family had no trace on her at all...
