One of the key muscle groups used in the sport of swimming is the shoulders.
Therefore, performing strengthening exercise for this muscle group is going to be extremely critical to your success in swimming.
One of the best shoulder exercises to perform that will help you with your swimming performance is an overhead dumbbell press. To execute this exercise grasp a pair of dumbbells, one in either hand. Then, ensuring your back stays completely straight, lift the weights so they are right beside your ears. Next drive them up straight above your head until your elbows are fully extended. Hold at the top of the movement and then slowly lower them back down to ear height once again. Repeat this process for 8-12 more reps.
A second exercise that will really help you is front and lateral raises. To do this one you simply hold a 5-10 pound dumbbell in either hand and then, keeping elbows straight, raise both arms either straight up in front of you or else directly to your side (for lateral raises). This is going to work either your front or lateral deltoid muscle. Take note that you should never move past shoulder height though because this will put additional strain on the muscles and joint and could risk injury.
Finally the last exercise you may wish to perform is a reverse fly and it will target your rear deltoid and back muscles. To do it lie face down on a bench, both arms straight below you grasping a pair of dumbbells. Next, with elbows slightly bent, move the dumbbells so they are forming a reverse arc with palms facing down, out to the side of your body. While doing this think of squeezing your shoulder blades together to ensure you are targeting the right muscles.
Always remember as well that it is important to stretch and warm-up well before you go out for a practice in the water. Taking these preventative measures, both with the stretches and exercises is the best way to not only improve performance but also decrease injury.