James Cameron is genuinely a world expert on submersibles. He made The Abyss and Titanic because he loves the deep sea

He made Titanic solely as an excuse to go down to the Titanic and film the wreck. He didn't even have an idea for the story when he pitched the movie

Then he became the first person to ever design and dive his own submersible to Chellenger Deep, and only the second ever submersible to reach that depth (after the US Navy's Trieste)