Map from Google - Ishigaki

The climate of Ishigaki is sub-tropical, with very mild winters, and long, moist and rainy summers.
Ishigaki is the most important of the Yaeyama Islands, an archipelago belonging to the southern Ryukyu islands, located to the east of Taiwan, and which is administratively part of Okinawa prefecture. Other islands of the Yaeyama archipelago are Iriomote, Taketomijima, Kohamajima, Hatoma, Yubujima, Hateruma (the southernmost inhabited island of Japan), and Yonaguni (the westernmost). The latitude of these islands is very low: just north of the Tropic of Cancer. Close to the islands is a coral reef.
Here are the average temperatures.
Average temperatures - Ishigaki
Min (°C)161718212426282726242118
Max (°C)212224262830323231292623
Min (°F)616364707579828179757064
Max (°F)707275798286909088847973

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 minimum temperatures about 10 °C (50 °F).
The long summer is muggy, with fairly frequent, and sometimes abundant rains. The first part of the rainy season, called the plum rains (Baiu), arrives 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, Ishigaki 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 - Ishigaki
Prec. (mm)1301401301552052051302602602041551252105
Prec. (in)

Kabira Bay

The sea in Ishigaki is never cold, in fact the temperature drops to a minimum of 23 °C (73 °F) in February, so if you want you can try to swim even in winter, while it gets very warm in summer.
Sea temperature - Ishigaki
Sea (°C)242324252728292929272625
Sea (°F)757375778182848484817977