I tried to do some research on tips for shooting in Galapagos and found some good ones in multiple sites but thought I would in my two cents as we just recently returned from Galapagos this week.
Here's what I brought:
Basically, I only used the 28-300L the whole trip. I'll be honest, it was a bit heavy to have to carry around but I found the results worth it. With the crop factor of my camera factored in, this ended up being 44-480. In my opinion, I found nearly nothing worth needed a super wide angle lens. I did find that the extra reach allowed me to get super close to the animals and frequently isolate their heads/faces if I chose to. While several people on our boat had their SLRs, none of them had the reach that I did, hence I earned the name paparazzi for the trip. This came in especially handy when we were following a family of orca's and also when we saw tortoises in the highlands. Because of the muddy conditions from the rain in the morning, we could only get so close to the tortoises but I got some great shots.
I would recommend bringing a polarizing filter if possible. Fortunately for me, it was overcast for many of the days because I discovered that my polarizing lens was broken the day before I left. Also, although I don't have one, a warming filter might give some nice effects. I say this because the sunglasses that I wore had this type of lens and I thought the colors really popped in some areas.
Also, shoot in shutter speed priority mode. This was one mistake that I made a few times. The IS on the 28-300 is pretty good and I didn't catch this because even on my 13" laptop LCD they looked good. However, when I got home and looked on my 27" IPS, I noticed a number of shots that were not tack sharp. I did notice that I didn't always follow the general rule of min shutter speed equal focal length. I shot our chasing of the orca's at 1/1000 and I would say that 99% of those pictures came out fantastic. Nearly all your shots will be during day light hours, which provides plenty of light so I would go with 1/500th at a minimum.
Hope this helps!