If I were you I would just leave your employer and not address the matter - then see if they choose to sue. My experience with those clauses is it's rarely, if ever, enforcable - or even worth the effort to pursue litigation over.
As a youngster out of college I'm guessing it's some amount under $10K - I doubt very much it would ever be an issue.
Simply resign, don't discuss the issue if your employer brings it up - and get on with life in the Navy .