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, which is located to the east of Taiwan, and administratively is 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

Ishigaki	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Ishigaki	J	F	M	A	M	J	J	A	S	O	N	D
Min (°C)	16	17	18	21	24	26	28	27	26	24	21	18
Max (°C)	21	22	24	26	28	30	32	32	31	29	26	23
Min (°F)	61	63	64	70	75	79	82	81	79	75	70	64
Max (°F)	70	72	75	79	82	86	90	90	88	84	79	73

Although the 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 around 10 °C (50 °F).
The long summer is muggy, with fairly frequent rains, and sometimes abundant. 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, 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 daily sunshine hours.
Sunshine - Ishigaki

Ishigaki	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Ishigaki	J	F	M	A	M	J	J	A	S	O	N	D
Sun (hours)	3	3	4	4	5	7	9	8	6	5	4	3

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 October, with a peak from August to early October, 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

Ishigaki	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Year
Ishigaki	J	F	M	A	M	J	J	A	S	O	N	D	Y
Prec. (mm)	130	140	130	155	205	205	130	260	260	204	155	125	2105
Prec. (in)	5,1	5,5	5,1	6,1	8,1	8,1	5,1	10,2	10,2	8	6,1	4,9	82,9
Days	18	17	17	15	18	18	16	18	17	16	17	18	205

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

Ishigaki	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Ishigaki	J	F	M	A	M	J	J	A	S	O	N	D
Sea (°C)	24	23	24	25	27	28	29	29	29	27	26	25
Sea (°F)	75	73	75	77	81	82	84	84	84	81	79	77