Waikiki is the famous one — two miles of sand, gentle waves, the iconic Diamond Head backdrop. It is crowded but genuinely good swimming, and the waves make it suitable for learning to surf. Kailua Beach on the windward side is broader, calmer, and significantly less crowded — arguably the finest beach on the island. Hanauma Bay is the best snorkelling, in a protected volcanic crater with an abundant reef (reservations required weeks in advance).
Makapu'u on the east coast is for experienced bodysurfers and lighthouse walkers rather than casual swimmers. Waimea Bay on the North Shore transforms from a calm summer swimming beach to a massive-wave surfing venue in winter. The Ala Moana Beach Park, adjacent to the shopping centre, is where Honolulu residents swim — calm, protected, and a world away from the tourist density of Waikiki.