I heard it from a person of Okinawa,
The best season of Okinawa seems to be October.
As for October, sea water temperature seems to be good for sea bathing at right temperature.
Temperature seems to be most suitable, too.
The sightseeing spot which I recommend.
"Okinawa Churaumi Aquarium"
http://www.kaiyouhaku.com/en/index.html
Scenery of here is splendid.
Okinawa SOBA is very delicious,
Please eat by all means.
山原そば(本部)