“If you want me to be more specific: hair that is not covered at all, knees, upper arm above the elbow, mixed swimming, shorts.”
The only one of these that I would say is for sure asur regardless of community, is “hair that is not covered at all.” Knees and above (i.e. shorts) according to the Mishna Berura is universal but the Shulchan Aruch doesn’t hold that way. Upper arm is b’pashtus dependent on the community. And mixed swimming, while an exceedingly bad idea, is not the subject of a particular Rabbinic prohibition as far as a I know.