Shoulder strength and flexibility are the two things that will always be the limiters in swimming along with having enough time to swim a lot and the boredom of swimming a lot. If you don’t have a long swimming history and haven’t built up significant durability and pliability in your rotator cuff muscles yet then these exercises can be very beneficial.
Below a few photos to help you work these muscles very thoroughly. If you take the cords with you when you travel then you’ll always be able to work on your swimming a bit. I make these exercise cords myself but you can buy them cheap enough in sporting goods stores.
In general the best exercises to prevent shoulder problems are rotator cuff strengthening exercises with cords and hanging from a bar to stretch. If you are short on time then just do 3 x 15 of internal rotation and 3 x 15 of external rotation and 3 x 20 seconds hanging from a bar or the top of a door (close to he hinges) if you don’t have a bar handy.
So do one set of internal rotators, one set of external rotators, then hang to stretch and go through that sequence 3 times. If you have more time then you can add swim simulation pulls as well.
Lateral Shoulder Raise with Cords