I'm a regular swimmer, and I usually do 6-8 sessions a week in a squad.
Shoulders: Hold two medium weights (I can't specify weights because I do not know your capacity) by your sides, with both arms fully relaxed. Slowly raise them to make a T shape (or the Jesus pose if you must), and hold that T shape for 30 seconds. Lower them down slowly, then have 30 seconds rest. Rinse and repeat.
I can't really help you with back. We never use weight training for back.
What we do however, is lie on our stomachs with arms outstretched forward like a superman pose. Try to raise your arms and legs whilst keeping them straight, so from a side view, it looks like you're trying to imitate a bowl. Hold for 30 seconds, then rest 30 seconds etc.