The Galapagos Islands are located at the Equator so there is not a lot of seasonal variation. Air temperatures tend to run in the 70s-80s year round, slightly warmer from January-April. The climate is relatively dry, though the majority of precipitation falls from January-April as well.
Water temperatures tend to vary significantly as the cold Humboldt current comes up from Antarctica from approximately May- November. Expect water temperatures in the 60s to low 70s during this season and in the 70s the rest of the year. Keep in mind that this also means that snorkeling is coldest during this season so the opposite months (December - April) are best for travelers snorkeling without a wet suit. The Humboldt current is strongest in September and October, so the water will be a little more choppy during these months
In general, you can visit the Galapagos year-round and have a wonderful experience! TIP: Booking in advance is always highly recommended for the largest selection of available yachts.