As with running, it's important to adequately warm your body up before you hop in the water. Just stretching your arms isn't going to cut it.
Here are some swim-specific movement prep exercises that I do before I swim to make sure my body has the mobility it needs to get through my stroke efficiently. This sequence only takes about three to five minutes.
Lunge forward with one leg and reach the arm on the side of the back leg straight overhead and slightly backward to open up the anterior chain—including abs and hip flexors. You'll also open up your quads and lats. Be sure to keep hips parallel to the ground, your abs and glutes engaged and hips/knees/ankles in line. Do four to six on each side.
This movement opens up the chest while warming up and strengthening the rotator cuff. Standing in a split stance (step one leg forward about two feet) raise arms out to their sides, parallel to the ground, turn thumbs toward the back, and engage (squeeze) the shoulder blades in toward the spine and slightly downward. Hold for one to two seconds, then release and repeat six to 10 times.
Increase your rotational stability with this movement. Lie on the ground on your side with legs on top of one another. Keep your lower leg straight but bend the upper leg so the hip and knee are both at approximately 90 degrees. Extend your arms out in front of you—to the side you are facing—so they are perpendicular to your body and your palms are touching. Press your bent knee into the ground and maintain it there while you take your upper arm up and over toward the opposite side, rotating the spine as you go.
The key is to engage your abs and glutes while keeping the bent knee on the ground so you can reach as far as you can with your spine (led by your arm). Ideally you will end up with arms, shoulders and back all flat on the ground, arms in a T, with the knee still touching the ground. Hold the stretch for a couple seconds, then return to your starting position and repeat four to six times on each side.
4. Your Choice of Either:
a. Quadruped Rocking With Back Flexion/Extension
Get on the ground squarely on your hands and knees. Sit back into your hips while keeping your abs engaged. Return to starting position, flex and extend your spine like a cat stretch. Repeat six times.
b. Standing Back Flexion/Extension and Lateral Extension
While standing, fully flex and extend the spine by doing a forward and backward bend—two to three times. Follow that with lateral bends by crossing one foot in front of the other and reaching the arm of the back leg up and over toward that back foot, creating a side bend. Hold for one to two seconds and repeat four times on each side.