These photographs of John F. Kennedy sailing are well-known. I like his natural sense of relaxed style and the combination of white with different shades of blue. Here in the Northern Hemisphere, summer is fading, but there's still time to wear those white or pale khaki chinos, that cornflower blue polo shirt and pale blue Shetland wool jumper if it's cool. Just let the clothes hang as you put them on - there's no need to tuck in your shirt, it'll only come undone as you rush around the foredeck repacking the spinnaker.
White canvas deck shoes are ideal; leather sailing shoes an acceptable alternative. White socks are fine, but a bright blue pair, to match your shirt, the sea and sky, are ideal. Wayfarers or similar sunglasses, with a heavy black or tortoiseshell frame, will keep the sun out of the eyes. Polaroids are best for cutting down the glare from the surface of the sea.