March to August is the season — warm sun, clear water, calm seas. May and June see the longest hours of daylight and the best snorkelling visibility.
September to November is the typhoon window, when storms can wash up from the South China Sea and leave the beach grey and rough. December to February is the cool, drizzly half of the central Vietnamese year — fine for the road trip over the Hai Van Pass, less appealing if you came for the sea.