It depends on who else would've/could've taken his place. Believe it or not, Tom Cruise was one of those originally considered for the part of Jack Dawson.
As for whether or not it would've bombed, who knows? I can assure you that the rates would've been different, but that's all I can say. IMHO, I think that about 60% of Titanic's success was due to Leo, but, out of the remaining 40%, 35% was due to Titanic curiosity and special effects.
Depends on who it was, I guess. There's no doubt a lot of teenage girls went to see it because of Leo. But I do cringe whenever someone says something like "the only reason Titanic was so successful was because of little girls going to see Leo over and over again." That's ridiculous. The movie was successful because it appealed to such a huge demographic. I saw people that looked like they were in their '80s in the theater when I went. I doubt they'd been out to a movie in years. The hype that surrounded it after it started getting good word of mouth and making piles of money didn't hurt.
Add to that the fact that it was released during a time of year when there weren't really any other real blockbuster competitors. It had clear sailing...so to speak.
Anyway, Leo's appeal was a big part, but if they'd gotten someone else who was a competent actor and looked OK, I'm sure the movie would've still been a hit, just not quite as big.