Monsoon seasons, and the island closures

The east coast of the peninsula (Perhentian, Redang, Tioman) effectively shuts from about November to February when the northeast monsoon brings rough seas; resorts and boats stop. The west coast (Langkawi, Penang) is better in those months, and Borneo is wettest in December and January. Don't plan an east-coast island trip in the winter, and expect afternoon downpours year-round: they pass in an hour.