On St. Kitt's there's basically 3 beaches "with facilities":
Frigate Bay - Caribbean side - relatively calm waters, several beach bars, within walking distances of several restaurants, shops & resorts, including the Marriott, some water sports available.
South Friars Bay - Caribbean side beach with slightly wavier water at one end. Home to the Famous Shipwreck Beach Bar, 2 other beach bars and the upscale Carambola Restaurant.
Cockleshell Beach - calmes waters, choice of two beach bars and a somewhat upscale restaurant plus one or two beach shacks serving a variety of refreshments, lots of water sports, horseback riding.
Other beaches without facilities include Whitehouse Bay, Turtle Beach, Major's Bay, and Sandy Bank Bay.
On Nevis, the calmest beach is Oualie Beach. Other Caribbean side beaches (all much calmer than any on the Atlantic side) include: Pinney's Beach, Paradise Beach, Nelson Springs Beach, Cades Bay beach (aka Chrishi Beach), Tamarind Bay and Jones Bay. Other beaches include Lover's Beach, Nisbet Beach, Herbert's Beach, Camps Beach, Windward Beach, Black sand Cove and Gallows Bay.