First thing to consider, we are in Ecuador. That means that the Galapagos lie directly on the Equator, and because of that, you will enjoy a warm temperature most of the year.
The hottest season is usually from December to April, but is also the rainiest. It is sunnier, warmer and more humid, but there are more chances to experience some heavy rains. Those rains usually don´t last more than 30 minutes, and the sun will shine again. Ocean temperature during this time is usually warmer
The coolest season is from May to November, and it is also the driest. During this time you can expect colder winds and very little rain. The water temperature during this time will be colder.
For a scientific background of the Galapagos weather, we recommend to check the page of the Charles Darwin Research Station:
Our friends of IGTOA has done also a very good job describing each month in terms of weather, so we highly encourage to have a read of it:
Weather & Climate of the Galápagos | IGTOA | Galápagos Islands Travel & Tours
In conclusion, you can visit the Galapagos all year around, and you will always experience wonderful days with enjoyable temperatures!