No, if it were made to be a love story, he would have started with just 2 people......
He started with the Titanic as a base for the movie, he's even said in interviews that he needed a fictional love story to draw the public's interest.
The movie is about the Titanic... the love story in it, which DOES appear to overshadow the real story, is just a doorway for Cameron to show what the story is actually about. Afterall, how boring would a movie be if it were ONLY about the ship. Even Titanic documentaries have people and characters in it. I've never seen a single movie about the Titanic where it was ONLY showig scenes from the ship, with no human conversation what so ever.

He didn't make countless dives and spend 2 years prior to the start of filming to extend his knowledge of love. He spend that time researching the Titanic... if the sole reason for the movie was love, he wasted alot of time and money making that point.

Yes, there is a love story in it, whether it was true love or not doesn't matter. But the sole purpose of that film wasn't to convey a love between 2 people, it was to shed light into the actual historic event.