Okinawa and the Southwest Islands experience a subtropical climate, and are much warmer than mainland Japan, particularly further south.
If you travel in the winter months of November to March, crowds are smaller, accommodation is less expensive and underwater visibility for divers is at its best. However, inter-island ferry services are culled outside the busy summer months.
Summer can be hot and crowded depending where you go. Typhoons, which can turn up at any time between June and October, wreak havoc with ferry schedules.
The average daily temperature in Okinawa in December is 20°C, while in July it is 31°C.