The division is anatomical, not a trick of posture. Each eye is split by the rim of the head into a dorsal portion above the surface film and a ventral portion below it, giving four functional eye surfaces in total. The two halves see different media, and the underwater halves are adapted to the different refractive conditions there.
The swimming is equally specialised. The middle and hind legs are shortened paddles that beat at high frequency, and the beetle is among the fastest surface swimmers of any insect, capable of tight turns while remaining in the film. Groups swirl in dense rafts that make individual targeting difficult for predators.
Whirligigs also read the water itself. They generate surface waves as they move and detect the reflections with the antennae, which rest on the surface film, using the echoes to avoid obstacles and one another. The system is often compared to echolocation, though it works in surface ripples rather than sound in air.