Due to its tropical climate, the Marieta Islands, including the Hidden Beach, enjoy 12 months of sunny days. Nevertheless, it is best to visit the beach during winter months, after the rainy season, when the waters are at their clearest and most beautiful. The best way to see the Marieta Islands would be to book a flight to Puerto Vallarta and then to find a tour operator that will take you to the islands. Travel comparison site Momondo have a useful page to help with comparing flights to Peurto Vallarta, where you can compare prices and find information on local living costs.
Sources: Huff Post, whenonearth.net, Daily Mail, Born Rich