I have joined few sessions of swimming class by different instructors, none of them stress the importance of stretching exercise. The instructors themselves also never do stretching before getting into the pool, at least I have never seen them doing so. It was ok during my first two sessions as I was only been taught of floating, buoyancy, and recovery from floating position in water. However, in the recent two lessons which I attended, I always suffer from shoulder ache, knee pain and groin muscle pain. Initially I thought I was doing the kick in an incorrect way which causes the pain. The coach has assured my technique but yet the pain always appear when I try to do the breaststroke kick. He asked me whether I have any history of bone injury or falling, but I could not recall any, especially when the pain is mostly from the groin muscle during I do the frog kick.
After doing some research online and asking opinion from friends, finally I realized the pain is formed because I did not conduct any stretching exercise! Swimming is a form of cardiovascular exercise and we need to use all our body muscles to perform this activity. Besides, water is a lot more denser than air, thus we need much more energy to exercise and move around in water. If we need warm up exercise for Zumba dance, aerobic dance, body combat, yoga, then warm up stretching exercise before swimming is even more important!
In order to prevent further injury, I will make sure stretching exercise is done before and after swimming. Below is a good site which teaches a few types of stretching exercise, hope it helps.