Palawan has two clear seasons: a dry season from November to May and a wet season from June to October. The peak tourist period is December to early March, when the weather is dry, the seas are calm, and Manila escapes the smog by flying south. Prices rise sharply over Christmas, New Year, and Holy Week (Easter).
April and May are dry but very hot — comfortable on a boat, less so in town. The early-shoulder months of June and November are reasonable gambles: still mostly dry, far cheaper, fewer crowds.
July to September is the height of the southwest monsoon. Boats are often grounded by the coastguard for days at a time; island-hopping tours can be cancelled at short notice; some resorts close entirely. Diving in Tubbataha is impossible — the only liveaboard season is mid-March to mid-June.
For the underground river and Honda Bay, the calm dry season is the easiest. For surfing on the east coast (a niche Palawan activity around Aborlan), the wet season is paradoxically the best window.