The dry season (December to April) is peak time: reliable sunshine, calm seas, and the best conditions for diving and beach use. December and January are busiest, with prices to match. February and March offer the same weather with fewer crowds.
The rainy season (May to November) brings afternoon and evening storms but typically clears by morning. The upside is that dive visibility around the Catalina Islands improves, giant manta rays arrive in numbers between May and October, and prices drop significantly. The coast stays green and lush rather than dry and brown. Surf on the Pacific increases in the wet season, though Flamingo's sheltered bay stays relatively calm.
Avoid Easter week (Semana Santa) unless you enjoy large crowds — the beach fills with Costa Rican families and prices spike.