It could be a number of things, though, I think she said she "used to," as if she no longer has an interest in marine biology. I could be wrong, I remember the scene but not the exact dialogue context.
Either way, its just a future plot point that will be worked into her character. Though, by looking in terms of character development - it appears Leaf, Serena, and Misty have been given the most character development and thought as of this time in the game's production.
Another individual on the forum brought up a similar question earlier in the context of Leaf's fear of water. You might find some of their observations to be comparable or contrasting with your own.
viewtopic.php?f=4&t=282