The climate of Okinawa is sub-tropical, with very mild winters, and long, moist and rainy summers.
Okinawa is an archipelago that includes the main island of the same name and some smaller islands (Kumejima, Agunijima, Iejima etc.). The nearby archipelagos of Kerama, Daito and Iheya have a similar climate.
Here are the average temperatures in the capital Naha.
Average temperatures - Naha

Naha	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Naha	J	F	M	A	M	J	J	A	S	O	N	D
Min (°C)	15	15	16	19	22	25	27	27	26	23	20	16
Max (°C)	20	20	22	24	27	29	32	32	30	28	25	21
Min (°F)	59	59	61	66	72	77	81	81	79	73	68	61
Max (°F)	68	68	72	75	81	84	90	90	86	82	77	70

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 and February, there may be very cool or slightly cold days, with maximum temperatures around 15 °C (59 °F) or below, and minimum temperatures around 10 °C (50 °F) or below.
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 than in the main islands of Japan, so it occurs in May and June: not only the rains are frequent, but also the sun is seen quite rarely.
Here are the average daily sunshine hours.
Sunshine - Naha

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

In July, there is a relative pause, so that the rains decrease, and sunshine becomes pretty frequent (although this does not happen every year, for example, in 2015, June has been quite dry while July has been very rainy). In the second part of summer (August, September and early October), the rains increase again, but the sunshine amount is generally decent, and certainly better than in May and June.
Here is the average precipitation.
Average precipitation - Naha

Naha	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Year
Naha	J	F	M	A	M	J	J	A	S	O	N	D	Y
Prec. (mm)	105	120	160	165	230	245	140	240	260	155	110	105	2040
Prec. (in)	4.1	4.7	6.3	6.5	9.1	9.6	5.5	9.4	10.2	6.1	4.3	4.1	80.3
Days	13	12	14	12	13	12	10	14	12	10	10	10	141

From June to October, with a maximum from August to early October, Okinawa can be affected by typhoons, the tropical cyclones of Southeast Asia. Of the 26 typhoons that form on average every year in the northwest Pacific, 3 or 4 pass close enough to Okinawa to threaten it or at least to have an impact on the weather. On September the 22th, 1999, the island was hit by Super Typhoon Bart, which unloaded 700 millimetres (27.5 inches) of rain in two days and caused severe damage due to high winds.



The sea in Okinawa is never cold, in fact the temperature drops to a minimum of 22 °C (72 °F) in February and March, while it gets very warm in summer.
Sea temperature - Naha

Naha	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Naha	J	F	M	A	M	J	J	A	S	O	N	D
Sea (°C)	23	22	22	24	25	27	29	29	28	27	25	24
Sea (°F)	73	72	72	75	77	81	84	84	82	81	77	75

The cherry blossoming in Okinawa occurs very early, even in winter, and is longer than in the main islands of Japan: two or three weeks pass from the beginning to full flowering (which occurs on average on the 1st of February).