Check out some of these off-season suggestions:
Swimming World - Seven things to think about during your off-season
Georgia Tech Swimming Off-Season Training - video from stack.com outlining some areas of focus
Off-season Strength Training - 6 tips on building muscle from active.com
Training with Stretch Cords - swim-related exercises to help with each stroke from active.com
USA Swimming video - plyometric training to help with strength and posture, and lower injury risk.