Kauai has the longest sand beach in the islands, as well as the longest coral reef. Many of its dozens of beaches and coves are accessible only by hikes. Generally speaking, in the winter, the north side beaches near Hanalei are pounded by the trade wind's swells and the south beaches of Poipu are relatively calm.
In the summer, the opposite is true, as southerly Kona winds bring bigger surf to the south and the north shore's coves become aquamarine pools. Beaches on the west side and east are variable in terms of water conditions — but beach conditions everywhere can vary greatly from day to day. Tip: Waves are like thugs, don't turn your back on them.