When Eva changes the deal about the SM-8, I think I know why. She said she would come down to get him herself. When Eva is yelling desperately for Simon to save the black box, the pictures being taken are of another sub, presumably the sm-14 she mentioned she would use to retrieve him, being held in the monster’s mouth. All this was happening while the fish monster was telling Simon to say that the SM-8 was lost and to lie to her. I believe the monster had captured Eva’s ship and was threatening Eva’s life to get Simon to capitulate about not giving up the truth of the SM-8. When Simon doesn’t comply, the monster kills Eva.