In the book I am reading at the moment "Menopause A-Z Staying in Control" by Joe Kabyemela MD she says that hot baths are not a good idea when wearing the patch and says it interferes with the way the estrogen patch works.
She suggests showering and having a bath on the day you change the patch. As I don't have a shower and bathe daily , I put the patch higher so it is above the water line.
I think the top of the arm is best for me as I very rarely go sleeveless. Also as the directions for applying Estrogel is applying on alternate arms from shoulder to wrist I can't see what harm there is, the principle is the same.
If you go to Hystersisters some of the ladies say that their surgeons place a patch on the back of the shoulder while in the operating room after the op.