I currently live in the capital, Naha. Just had a blast riding today, nice and steady 17knot!
Winter time we get pretty strong and gusty winds (from the North), the best season would be Fall time from October to December. Other good time of the year is from March to May though I haven't lived here long enough to confirm this.
Avoid summer time, extremely light winds unless a Typhoon swings by
Like ryuu said you need protection for your feet, tons of small coral shards. There are a few areas that are nice and sandy though.
Once you get out the main island there are amazing places to kite, including Ishigaki island, Miyako island and my personal favorite Kumei island which includes sand dunes in the middle of the ocean, you can kite any wind direction and its flat one side, choppy on the other, and has waves on another side.
For more info check out the local clubs here:
http://www.kiteclub.biz/ (japanese)
http://www.patriotkites.com/ (english)
There's a yearly festival/competition going on this weekend; I'll try to post some pics.
current wind in Okinawa:
http://www.windguru.com/int/index.php?sc=98381
Hope this helps!