The rotator cuff group is comprised of four muscles, commonly referred to as SITS.
Supraspinatus, Infraspinatus, Teres Minor and Subscapularis. Most of these serve to rotate the arm, although the Supraspinatus is responsible for abduction (movement away from the body) of the upper arm.
Caveats - All of the exercises listed should be done with light weights or band resistance. They are not especially strong muscles comparatively, and form is paramount. If done wrong, it would be easy to injure/worsen an injury.
Also, exercises should be done slowly, 2-3 seconds when going against gravity, 3-5 seconds when returning.
Here are a list of exercises that can help strengthen the SITS group:
Side lying external rotation - Creme de la creme, hits all the muscles of the group. Lie on side, upper arm along body, lower arm across abdomen. (Like if your arm was in a sling). Simply rotate arm, keeping 90 degree bend, until hand is pointing at ceiling. Return to starting position.
Prone abduction - Kneel on bench, upper body horizontal to floor. Palm should face in towards body, raise straight out to side. Don't raise past horizontal to floor. Return to starting position.
Vertical variation - Same as prone abduction, but forward (So if you were standing, your hand would be above head).
Row with rotation - Same kneeling position, perform row until upper arm is parallel with floor, elbow should be at 90 degrees, lower arm still pointing at floor. Keeping upper arm parallel, rotate upper arm forward until it is also parallel with floor. Reverse entire motion.
For all of these, no weight or a section of broom handle for a grip, and see if there is any pain. If there is pain, it is possible that you have an impingement of the muscles or something else also going on. If no weight produces no pain, move to a very light weight or easy band.
Finally, while it's good to rehab and strengthen it, unless this is a chronic type injury because you've been swimming for years (like my shoulders, they're rubbish), it is possible that your swim form is contributing to this, and I might have a good stroke coach take a look and see if you have some stroke faults that are causing this. It'd do no good to rehab it only to re-injure.