Genetics! For science, of course.
In high school I really hadn't a clue what I wanted to do. Or rather, I did know I wanted to do something with the life sciences, but not what. Luckily, I had a good counselor who helped me focus on molecular biology/genetics. Why? Because it's life's alphabet- everything from a toadstool to a banana to a giant squid is encoded by DNA. Genetics is a more specific subject than "I want to study biology!", but on the other hand it has many broad applications- evolutionary biology, medicine, archaeology, etc.
Gradually, I've figured out it's not entirely what I want to do. While extremely fascinating, I'm not sure I want to follow specific chemical pathways and see that x secretes y- I want to know if that pathway will yield a distinctive physiological change in the organism for a particular reason. Forest and not the trees, I guess. While I jokingly tell people "Imma gonna do SCIENCE" when asked what I want for a future career, I think I'd much prefer working on the organismal, physiological side of things. I want to do research, which is a very broad, very grey unknown area but that's the fun part. If I had to choose a specificity, I'd probably want to investigate deep sea communities for unique behaviors and structures that are a) weird and b) potentially useful for human use.
So, yes, I think I'd need to know genetics to figure out what makes a tubeworm be able to live in very acidic, very hot water full of metal particles.
