A little bit if xanax might help...
When you ask her, what does she say about the diff between her normal adventurousness vs. this?
I think it's just the complete alien nature of the environment and a fear that if something goes wrong she won't be able to easily resolve the issue. Plus most of the trips are boat diving, and she is afraid of being so far offshore if she does freak out and want to leave. And I think the overall anxiousness she's feeling is leading to silly fears like sharks and rays. She is ridiculously bouyant, so they have to put a ton of weight on her to get her to sink, and that causes her to be afraid that she has too much weight and is somehow gonna forget how to control he bouyancy and she'll sink to the bottom.
None of her concerns are exactly rational, and I think she even admits that, but it's hard to get yourself to stop being nervous or anxious about something just through talking about it.
I doubt she'd go for the Xanax, but it's a good idea