I'm not a huge fan of Titanic, but still, I heard this argument -"we know how it ends!"- many times and I don't buy it. I can enjoy a WWII movie even if I know who wins, or a biopic (Buddy Holly dies at the end! So does Jim Morrison!) even when I'm fairly familiar with the events being covered. And then let's talk about movies that are based on well known fiction, like Lord of the Rings, I already KNEW how the third movie would end during the opening credits of the first one, and I still managed to enjoy the hell out of all three!
Titanic wasn't about the sinking ship but a young couple who fall in love and a tragedy that ends their romance.