I was there a couple of years ago. Many of my photos are posted at
http://jfiddler.smugmug.com/Galapagos. Use the info icon to see the focal length for each shot. These were mostly taken with a D300, so multiply focal lengths accordingly. Sadly, it doesn't tell you which lens, just the focal length.
Realize that you will be moving with a group. You MUST, by park regulation, stay with your guide and group, so you'll often find yourself working quickly in order to keep up with the group. Because of that, many of my images were taken with an 18-200 Nikon lens - not my "best" lens, for sure, but you can see that it was good enough, and extremely convenient. The animals are never afraid of you, so you can always get close, and rarely need a very long lens. (See, for instance, the hawk pics at
http://jfiddler.smugmug.com/Galapagos/Birds/Galapagos-Hawk/8401219_BxzKMc. I could have reached out and touched him.) I also used a 150-400, mostly for birds. I also used my 10-20 a fair amount.
Because of the need to move quickly, I used a monopod a lot. I tried my tripod just once, and then never carried it off the boat again. I'd also recommend taking a color reference. I love these
http://www.sjphoto.com/graycap.html, but something like the x-rite passport would also be great. I took gray reference photos every time I shot, and any time the light seemed to be changing. Equatorial light can be different.
I'd also recommend taking an underwater camera, just something like the Pentax WP-1. The snorkeling is spectacular, and you'll want to be able to record it. I took some underwater movies and those far exceeded my expectations. Having a little point-and-shoot like that is also great on the boat.
Have fun!
