I have made two trips to Galapagos - and I'll be taking my third trip later this year. I find the best lens for the wildlife is the Canon 100-400. For the few landscapes and seascapes I shot (after all, you go for the amazing wildlife!) I used the Canon 24-105. Those two lenses will cover at least 95% of your photo needs. The 24-70 f2.8 is perhaps a better wide angle fit in that it also offers passable macro capability.
I was able to capture everything from birds in flight from a panga bobbing on the ocean to the entire amazing panoply of land and marine animals.
If possible, be sure to take a camera for underwater. Fully 50% of the wildlife will be in the ocean. For my first trip I used a simple Canon point and shoot in a Canon housing. For my second trip I used an old Canon D60 SLR in an Ewa-Marine underwater housing.
Have a great time - I know you will.