Mythbusters proved it was buoyant enough. James Cameron has since clarified the physics: It wasn't about buoyancy, but buoyancy + thickness . Jack tries to get on, and the raft sinks lower, submerging him in freezing water. He chose to stay in the water to save her. Also, Rose is in shock, wearing a heavy wool coat, and likely suffering from hypothermia.
Cameron understood that we needed to care about the characters before the water starts rushing in. The first two hours are a slow dance of longing and rebellion, making the final hour of chaos almost unbearable to watch. Every epic needs a villain, and Cal Hockley (Billy Zane) is a masterpiece of entitled cruelty. He isn't a cartoonish monster; he is the embodiment of the oppressive Gilded Age. From putting the necklace in Jack’s pocket to that terrifying chase through the flooding dining room, Cal gives us someone to hiss at. Titanic Movie Complete
But the real answer is simpler: If Jack lives, the movie doesn't stick the landing. His death is the price of her survival. She promises to never let go, and she doesn't—she lives a full, adventurous life because of him. Titanic won 11 Oscars (tied for the most ever). It was the first movie to hit $1 billion. It made Leo a permanent heartthrob and turned "Draw me like one of your French girls" into an immortal meme. Mythbusters proved it was buoyant enough