“Just hanging around”
Swimming by its very nature places load on your shoulder anatomy, thousands of repetitions of movements extending, and lifting your arms above your head causes a lot of wear and tear. Strong flexible shoulders maintained with prehabilitation (injury prevention) exercises will let you swim into your old age as injury free as possible.
From an evolutionary standpoint we are an arboreal (living in trees) species. We evolved to hang, swing and climb. We still have those abilities. But we have forgotten how to use them, with many of us spending our time hunched over screens or desks. Incorporating hanging into our activity can improve flexibility, mobility, and alignment of the shoulders.
Your shoulders are supposed to move in all kinds of directions, including above your head. When swimming it is very important to maintain shoulder health, hanging from a bar or object (or door) brings everything into alignment and stretches out your latissimus muscles (the big wings on your back)
Rolling a ball on the wall in front of you with one hand helps strengthen the shoulder capsule, why not try rolling the letters of your name using a ball on the wall 5 to 10 times,try with one arm at a time, repeat for a few sets.
Stretch out by hanging off a bar or beam, or the top of a strong door, using a towel or pad on top if you have sensitive hands .Start with hanging for 10 seconds and build up the time, hang at full stretch you may have to tuck your legs up behind you depending how high your bar/beam is.
Repeat 2 to 3 times a week and see how your shoulder strength and health improves. Transfer that improvement into the water when you swim train, I guarantee you will feel a difference.