I don't recall the Outer Limits episode that Harlan claimed was plagiarized, but from the summary I read on Wikipedia the connection sounds pretty tenuous. Maybe a similar look and feel, but not the same story at all.

"How much of the success of the movie is his and how much is James Cameron (who later had tremendous success with Titatnic and Avatar) I can't say."

I'd say James Cameron.

It occurred to me after watching Terminator that Titanic is exactly the same story, only with a shipwreck instead of a killer robot from the future. Young couple is thrown together and have a whirlwind romance which is immediately disrupted by a cataclysmic and life-threatening emergency. Together they fight against seemingly insurmountable odds, taking turns saving each other, and occasionally finding time between gigantic action scenes for a few quiet and touching moments together. Just when it looks like they are going to make it after all, the young man poignantly dies in his beloved's arms. The young woman, having discovered a strength she didn't know she had, continues on alone to make a life for herself much different from the life she was living before.

It worked for me.