Hawaii Beaches get busiest in winter [Dec - Feb], mainly because of the weather elsewhere. June to October is the hottest beach weather, December to March the wettest.
The average temperatures are more or less the same winter and summer. Hotel prices are highest mid-December to the end of March.
If you're into surfing, winter months are the best, especially the Christmas season at Oahu's North Shore, otherwise the summer [July-August] has the best water condition for swimming, snorkeling, windsurfing and scuba diving.
Beware that some beaches have seasonal high surf in summer.
