Map from Google - Miyakojima

The climate of Miyako-jima is sub-tropical, with very mild winters, and long, moist and rainy summers.
Miyako-jima is the most important island of the archipelago of the same name (Miyako Islands), which is located to the east of Taiwan, and administratively is part of the Okinawa prefecture. Other islands of the Miyako archipelago are Irabu, Minna, Tarama, Kurima, Ikema, Ogami, Shimoji. Together with the Yaeyama Islands (see Ishigaki), the Miyako Islands are part of the Sakishima Islands, in their turn belonging to the southern Ryukyus. The latitude of these islands is very low: a bit to the north of the Tropic of Cancer. Near the islands is a coral reef.
Here are the average temperatures.
Average temperatures - Miyakojima
MiyakojimaJanFebMarAprMayJunJulAugSepOctNovDec
MiyakojimaJFMAMJJASOND
Min (°C)161618202325272625242118
Max (°C)202123252830323130282522
Min (°F)616164687377817977757064
Max (°F)687073778286908886827772

Although the winter is very mild, the sky is often cloudy, and periods of bad weather, with wind and rain, are quite common. Every now and then, especially in January, there can be very cool days, with highs around 16 °C (61 °F) or less, and lows around 10 °C (50 °F) or less.
The long summer is muggy, with fairly frequent, and sometimes abundant rains. The first part of the rainy season, called the plum rains (Baiu), begins one month earlier than in the main islands of Japan, so it occurs in May and June, however, between a shower and the other, the sun can come out, especially in June. In July there is a relative pause, so that the rains decrease, and sunshine becomes pretty frequent.
Here are the average sunshine hours per day.
Sunshine - Miyakojima
MiyakojimaJanFebMarAprMayJunJulAugSepOctNovDec
MiyakojimaJFMAMJJASOND
Sun (hours)334456876543

In the second part of summer (August, September and early October), the rains increase again, but the sunshine amount remains generally decent, at least until September. If all goes well, a week can pass without rain, otherwise there may be periods of bad weather, or even typhoons.
In fact, from June to November, with a peak in August and September, Miyako can be affected by typhoons, tropical cyclones of Southeast Asia, which can bring strong winds and torrential rains.
Here is the average precipitation.
Average precipitation - Miyakojima
MiyakojimaJanFebMarAprMayJunJulAugSepOctNovDecYear
MiyakojimaJFMAMJJASONDY
Prec. (mm)1301401401602101851302652301551451302020
Prec. (in)5.15.55.56.38.37.35.110.49.16.15.75.179.5
Days191718161818171917171819213

Miyakojima

The sea in Miyako is never cold, in fact the temperature reaches a minimum of 23 °C (73 °F) in January and February, so if you want you can try to swim also in winter, while it gets very warm in summer.
Sea temperature - Miyakojima
MiyakojimaJanFebMarAprMayJunJulAugSepOctNovDec
MiyakojimaJFMAMJJASOND
Sea (°C)232324252628292929282625
Sea (°F)737375777982848484827977