Alaminos and the Hundred Islands are best visited during the dry season, which runs from November through April. The sea is calm, visibility is good for snorkelling, and the risk of typhoons is low. March and April are the busiest months — Philippine school holidays bring large domestic crowds to the park, and Lucap Wharf can be overwhelmed on weekends.
The wet season (May to October) brings afternoon rain squalls and occasionally rough seas in the gulf. Boat trips are sometimes cancelled when swells are high, and visibility underwater drops after heavy rain. That said, the islands are genuinely beautiful in the low season, crowds are thin, and rates at the few guesthouses near the wharf drop considerably. If you come outside school holidays in the wet season, you may have entire islands to yourself.
Alaminos holds its city fiesta in March in honour of its patron saint, Saint Joseph. The celebrations are modest by Luzon standards — a procession, a fair near the cathedral, and a lot of noise in the evenings — but they add some colour if you happen to be there.