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 Okinawa prefecture. Other islands of the Miyako archipelago are Irabu, Minna, Tarama, Kurima, Ikema, Ogami, Shimoji. Along with the Yaeyama Islands (see Ishigaki), the Miyako Islands are part of the Sakishima Islands, in turn belonging to the southern Ryukyu. The latitude of these islands is very low: a bit north of the Tropic of Cancer. Close to 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 winter is very mild, the sky is often cloudy, and periods of bad weather, with wind and rain, are quite common. Sometimes, especially in January, there may be very cool days, with highs around 16 °C (61 °F) or less, and lows about 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 compared to the main islands of Japan, so it occurs in May and June, although, 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. 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, the tropical cyclones of Southeast Asia, which can bring strong winds and heavy 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 drops to a minimum of 23 °C (73 °F) in January and February, so if you want you can try to swim even in winter, while it gets very warm in summer.
Sea temperature - Miyakojima
MiyakojimaJanFebMarAprMayJunJulAugSepOctNovDec
MiyakojimaJFMAMJJASOND
Sea (°C)232324252628292929282625
Sea (°F)737375777982848484827977