The Galapagos Islands have a surprisingly cool, sub-tropical climate regulated by the cold Humboldt and Peruvian ocean currents orignating in Antartica. The two Galapagos seasons both have some precipitation.
Temperatures vary depending on the strength of the trade winds.
From December through March, the cooling currents subside and temperatures rise into the 80's and 90's. In April, both air and water tempertures begin to cool. In October the trend reverses again. Bring a pair of sandals and tennis shoes. For land excursions, a plastic poncho can come in handy for the visit to the highlands.