Normally I’d agree with you on a Hollywood figure, but Cameron is *actually* an expert - not a leading expert, but he’s got engineering and crew experience on submersibles. Here’s one of the craft he worked on: https://en.wikipedia.org/wiki/Deepsea_Challenger
He got into diving and submersibles in order to advance his filming. One of those “In order to advance my career in one field, I had to become an expert in another” or “I took up a hobby and became an expert in that field” kind of things - kind of like how Paul Newman was an actual expert on motor racing, for example.
Interesting. Thanks.