Kauai's weather is usually localized meaning that you're likely to see scattered rain showers and sunshine in the same day - especially as you move around the island. Generally there is more rainfall on the north shores of Kauai than on the south. As a result the vegetation on the north is much more green and lush while some areas on the south shore are arid. Kauai's temperature is mild and pleasant throughout the year with only a small temperature difference between night and day.
There is no "rainy season" in Kauai. Although there is generally more rainfall during the winter, there is not a season when you can expect prolonged periods of rain. Generally, when it's raining in one area, you can drive 10 minutes and find sunshine somewhere else.
Water temperatures tend to average around 80 degrees in the summer and 73 degrees in the winter.