Kona is the dry side of the Big Island and the weather is reliably warm and sunny year-round. Daytime temperatures sit in the high 20s Celsius almost every month; sea temperature stays around 24–26°C. The wetter season is November to March on the windward side of the island, but Kona itself stays mostly dry.
The most popular time is from mid-December to mid-April, when North American travellers escape winter. Whale watching is at its peak in February and March. October is the busiest single month for Kona itself, because the Ironman World Championship triathlon is held on the second Saturday — book accommodation months ahead if you want to be in town for it. May and September are good shoulder months: warm water, lower prices, no crowds.