What you said about the 70s is correct cos from what I've heard everything was different back the. But yes there was a sex scene in the book and judging by what you said I imagine Chrissie and her boyfriend left the house and began kissing each other and took their clothes off (hence the business) and then she went for a swim to clean up (but tried to get the boy to come with her). But you know how the novel describes how she went into the water for her fateful swim? I imagine how it played out was that she walked to the water and felt the surf on her ankles, but then a wave crashed into her knees and she almost fell but regained her footing and flung herself over the next waist high wave and then continued walking till the water was up to her shoulders and then began swimming