Here in Sydney, Australia, the best months are undoubted in the Spring and the Autumn (Fall) Summer is generally too hot, winter just a little bit cool. Mind you, the whole year is pretty mild compared with many countries in the world. My own favourte month is the Autumn month (Autumn in Southern Hemisphere) of April.
April in Sydney is usually blue skies, hardly any wind, midday temperature 75 degrees Fahrenheit. The sea temperature is generally at it most warm point, and with the long Easter Break, it's ideal for swimming or surfing in the sea.