I'm a little confused by your question. It's fine to say you really want to work on a sub, in which case you might want to be a Sonar Op or Weapons Engineering Technician, because those are the most numerous trades on a sub.
However, to say it's because you really want to do ASW...then you're in for a bit of disappointment. Not a lot of ASW going on these days, although it's still practiced I guess. That's like saying you want to be a CF-18 pilot but are really only interested in precision bombing runs.