The dry season, from November to May, is the only practical time to visit. Boat services are reliable, tracks are passable, and the weather is clear. December to February is the sweet spot: cool and dry, turtles still nesting, hippos active, and migratory birds present in large numbers.
Avoid the rainy season (June to October). Humidity is extreme, boat services become irregular or stop entirely, and some islands are effectively inaccessible. The mangrove mosquitoes are ferocious year-round; bring strong repellent and consider a mosquito net for the guesthouses.