The Galápagos has two distinct seasons, and both have their advocates. From December to May the warm season brings calm seas, clear skies, and warmer water (around 25°C). The vegetation turns green after the rains, sea turtles nest on the beaches, and snorkelling conditions are excellent. This is also high tourist season, particularly around Christmas and Easter, so prices are higher and boats fill up faster.
From June to November the cool, dry season arrives with the Humboldt Current, pushing water temperatures down to around 18-22°C and making the sea choppier. This is harder on casual swimmers and less-experienced snorkellers, but the wildlife activity intensifies dramatically. Seabirds begin nesting in earnest — blue-footed boobies perform their courtship dances, frigate birds inflate their pouches, waved albatrosses arrive on Española. Marine life visibility tends to improve in the cooler, clearer water.
For Kicker Rock specifically, the dry season generally offers better diving and snorkelling conditions: the currents are stronger (which concentrates the marine life) and the water is clearer. The trade-off is a rougher boat ride out. If your main priority is snorkelling or diving at Kicker Rock, aim for July to October. If you want calm seas, warm water, and green scenery, December to March is the better choice.