For islands like Perhentian, Redang and Lang Tengah, Tenggol, and Tioman, the most reliable window is usually March to October. Seas are typically calmer, island resorts operate full schedules, and visibility is often better. From roughly November to February, the Northeast Monsoon can bring rough crossings, more rain, and cancellations. If you must travel in that period, plan a backup coast like Langkawi or Sabah, and book refundable stays where possible.