Chrissie swims out into the bay but, unlike the image of her on the poster, she keeps her head above water as she executes a clumsy crawl stroke. In a wide shot we see how inviting a midnight swim could be: the water has a silvery sheen to it (an effect achieved in post-production as these night scenes were filmed during daylight hours) and the only sounds are of the waves and the occasional bleat of a buoy as it is gently rocked by the tide. On the horizon to the left and the right of the frame there are low masses of land that presumably mark the mouth of a natural harbour. In the middle distance, slightly to the left of centre, is the silhouette of the buoy (looking rather like the RKO tower) canted a few degrees to the right.
Chrissie - a black outline on the surface of the water - is now swimming on her back from the right of the frame and after two strokes she draws her leg towards her and then extends it until it is perpendicular with her body. In synchronised swimming this is a basic position known as the Ballet Leg. With only her head and the extended limb visible, Chrissie slowly sinks below the water, keeping the leg perfectly straight until it is completely submerged.
The shot recalls images of Esther Williams, the star of a series of aquatic MGM musicals (including Million Dollar Mermaid, Neptune's Daughter and Dangerous When Wet). Her underwater acrobatics clearly inspired Julie Adams when she went for a swim in The Creature From The Black Lagoon, and it may have been this scene rather than the original Fifties musicals than gave Spielberg the idea of including the Ballet Leg figure. The shot of Chrissie's long shapely leg extended above the water is mildly erotic - it's almost as if she is teasing the viewer with a glimpse of thigh - but it also serves to deconstruct her from summer girl into shark victim. In the water she has already become simply the collection of body parts (arms, head, leg) that will wash up onto the beach.
The fact that Chrissie can execute a perfect Ballet Leg suggests that she - like Esther Williams - is an experienced and confident swimmer, completely at home in the water. Indeed, the ocean almost seems to call to her when she is seated by the campfire and she turns her head towards it. She even has the traditional long flowing hair of a mermaid.