First of all, it makes a difference as to what side of the island the beach you are planning to visit is located. As a general rule, if your beach is on the windward side of the island along its eastern and northern coastlines, there is a great possibility that the trade winds (which blow about 300 days a year) will bring in a varying amount of both breeze to stiff wind and a varying amount of showers, from misting to full on rainfall, at some point in your day. If you are visiting a beach on the western or southern coastline of the island the reverse is likely true and you will be experiencing hot or warm sunny weather with a much lesser likelihood of wind and rainfall.
During the winter months the reverse weather conditions may prevail when the trade winds are taken over by winds coming from the south called, Kona Winds, and the south and eastern shores will be sunny and without wind and rain and the west and southern shores will experience the rain and wind. Although that is the trend, keep in mind that the quantities of rainfall at the beaches on the windward coasts are much, much greater than the rainfall totals that will fall on the leeward side beaches. It is fair to say however that during the winter rainy seasons rain can be torrential at times along the leeward coasts as well but usually only for short periods of time. The time of year will produce variations in temperature as well as rainfall as mentioned above. In the winter the beaches on the leeward coastlines will get to about 80 degrees Fahrenheit during the day and perhaps reduce about ten to fifteen points in value into the evening. The entire island is extremely temperate and the difference in temperature in the winter months on the windward coast will only amount to about a two degree shift downward both in the daytime and for overnight temperatures. The peak temperatures in the summer for beaches on both sides of the island will increase by about six degrees and consequently you should find the beach temperatures quite delightful at any time of year.