The peak season in the Andaman Sea runs from November to April, when seas are calmer and visibility is best for snorkeling. May to October is the monsoon period: some national parks (including Similan) may close, but many day trips still operate with safety checks.
Quick answer: For Phi Phi and Coral Island, November–April is ideal. For fewer crowds, consider May or October shoulder weeks.
See our boat tours, fleet, and island guides.