Go see the sea paradise in Miyako Island (Miyako-Jima) in Okinawa!
Reason for Okinawa & Miyako Island
My wife and I make it a rule to visit Okinawa every summer, as it's a marriage promise that I made to her. She was living there for 4 years in her university days and enjoyed skin diving and snorkeling in the sea, so she wants to go back there every year.
We think the best attraction in Okinawa is actually the natural beauty in the sea; Okinawa is abound with transparent emerald green seawater, beautiful coral reefs and various colorful fish everywhere. They can be enjoyed easily by snorkeling not only off the coasts on a boat tour but also near the beaches/coasts, on its largest Okinawa Island (main island, 沖縄本島) and its several neighboring islands as well. Of all those snorkeling treasure places we recommend Miyako-Jima (Miyako Island, 宮古島) in particular, a 40-50 minute flight from Naha Airport on the main Island.
Yoshino Coast （吉野海岸） & Aragusuku Coast （新城海岸）
Both snorkeling and diving must be done at your own risk, but snorkeling is basically easier than scuba diving. You don't have to wear big and heavy equipment that you would need for scuba diving, and all you have to rent (or bring with you on your own) are a snorkel, a mask, and fins. You should also rent a life jacket/vest and gloves, etc. and ask for a guided insutruction tour if you are a novice first-timer or not a good swimmer.
The best snorkeling spots near the coasts are Yoshino Coast （吉野海岸） and Aragusuku Coast （新城海岸） on the east side of Miyako Island.
You can enjoy a video of snorkeling at Yoshino Coast here.
Yabiji （八重干瀬） Coral Reefs Area off the Miyako Island
Yabiji （八重干瀬） is also a popular snorkeling and diving area with huge groups of beautiful coral reefs, located about 3 - 15 miles to the north of Miyako Island. You can go there on a guided boat tour (conducted only in Japanese), so you may have to bring your Japanese-speaking friend or hire a translator with you. The tour (usually 5 hours with a lunch) departs from Ikema Island （池間島）, a small island to the north of Miyako Island, so in order to get to the departure point, you pass the Ikema Bridge （池間大橋）, which I say is Japan's answer to Seven Mile Bridge in the Florida Keys; it's much shorter but more beautiful.
For more info on Yabiji and its coral reefs, see the URL below: