Theo James nude scene was edited by PBS for US audiences. Instead of seeing him completely nude from behind when he emerges from the sea in front of Charlotte (as shot), the scene is zoomed in so that no nudity from his character is shown.
Theo James' nude swim is Andrew Davies' homage to Colin Firth's famous swim in Davies' BBC adaptation of Pride and Prejudice (1995). It is, however, not salacious as gentlemen in the period in question often did swim in the sea in the nude.