The best time to dive Bikini Atoll is between April and November, when seas are calmer and visibility in the lagoon is at its clearest — often exceeding 30 metres. The northern Marshall Islands sit outside the main typhoon belt, but swells and wind-chop from December through March can make the passage from Majuro uncomfortable and complicate liveaboard operations.
Because access depends entirely on live-aboard expeditions, the question of timing is largely settled for you when you book. Most operators run a small number of departures per year, typically in the May–October window. Check current schedules with the Bikini Atoll local government or established dive operators in Majuro well in advance; some seasons see only two or three expeditions depart.